docs(plan): record the Door C facet decisions, the mutation result, and the harness that lied

Four decisions: Door C projects and never derives (D-C1), the projection is
key-agnostic so it is not about numbering (D-C2), an unrenderable value drops
the facet and never the concept (D-C3), and a faceted entry is refreshed
because it is the first entry that can go stale (D-C4).

Also records a measurement about the measuring. The mutation harness first
reported 11 of 11 killed; the number was entirely false, because it invoked
pytest with an unrecognised `--timeout` flag and every run exited non-zero. The
negative control caught it by being reported killed when it provably could not
be. Honest result is 9 of 11, with both survivors analysed as equivalent
mutants and the one real gap they exposed (profile threading pinned only by
coincidence) closed with a direct test.

Closes open item 6 of the Door B record, which asked exactly the question this
answers: whose claim does the frontmatter make.

docs/plan/structure-derivation.md

@@ -256,7 +256,167 @@ change — rather than as "not possible".
    (`entries_match_directory`) in the same call, because `STRUCTURED_V1` does
    not set it. Not attempted; no consumer has asked.
 
-6. **No profile is applied to Door C.** Import writes concepts verbatim and
-   derives nothing, unchanged. Deriving structure for content another sender
-   wrote would put our inference into their bytes; if that is wanted it needs a
-   decision about whose claim the frontmatter makes.
+6. ~~No profile is applied to Door C.~~ **Closed** — see the Door C section
+   below. The objection stated here is the one the answer is built on: Door C
+   PROJECTS the sender's declared facts and derives nothing.
+
+
+---
+
+# Door C: the index carries the sender's facts, and only theirs
+
+Ordered by vegnormal-okf (`20260826T224500Z-873805419`) on a measurement, not a
+preference. A pre-registered reasoning bake-off on 2026-08-27 ran the same
+bundle through two index shapes:
+
+| arm | index shape | hits |
+| --- | --- | --- |
+| B | `DEFAULT` profile's `index.md` | **0 of 8** |
+| — | faceted index over the same frontmatter | **25 of 29** |
+
+Same bundle, same concept files, same model. The `DEFAULT` arm did not answer
+wrongly; it **abstained**, because the fact it needed was never in the file it
+was given. Measured on their `index.md`: 30 974 characters describing 269
+requirements, with **0 occurrences** of any of the eight facts (title, date,
+status, supersedes, erstatter, req_number, seksjon, kravtype) that the concepts
+in the same bundle carry.
+
+`FacetPolicy` and `STRUCTURED_V1` already did this. They did not reach Door C.
+
+## Decisions
+
+### D-C1. Door C PROJECTS, and never DERIVES
+
+Door B derives structure from documents this repo's own operator dropped. Door C
+merges documents a **third party** wrote, verbatim, and the entire door is built
+on refusing to put anything of ours inside their bytes.
+
+Running the deriver here would not have broken the verbatim guarantee where
+anyone would look for it. The concept file would still be byte-identical. It
+would have broken it in the index entry *about* the concept — where our
+inference is printed next to their name and reads as **their** claim. Forging an
+attribution is not a smaller failure than editing a file; it is a quieter one.
+
+So every facet in a Door C entry is a value the sender wrote in their own
+frontmatter, copied. Nothing is inferred from their body, their filename, or
+their neighbours in the bundle. Where the sender carries `derived`, **their**
+list travels unchanged — so a reader can still see which of the sender's facts
+the sender inferred, a distinction we would erase by adding inferences of our
+own beside them. That is the whole ownership stamp at this door: the concept is
+verbatim, and so is the index's account of what the concept claims.
+
+`resolve_structure` is deliberately not called. It computes supersession across
+a whole document set, and at this door that set is somebody else's.
+
+### D-C2. The projection is key-agnostic, which is why it is not about numbering
+
+The loop asks the policy which keys to carry and never what a key *means*. It
+reads a value only to check the policy can render it.
+
+Operator directive 2026-08-27, verbatim: *"alt vi lager rundt OKF publiseres på
+Forgejo og skal virke for alt av innhold"* — so a design that only works for
+numbered norms is wrong even when it scores well on N100/N200/N500. Nothing in
+`_project_facets` can key off a numbering scheme, and the suite says so with an
+unnumbered Norwegian meeting note (`status` + `date`, no number, no parent, no
+version) and with a document declaring none of the policy's keys, which renders
+the bare link rather than a separator with nothing after it.
+
+The same property pays a second time: a consumer whose concepts are named by
+UUID gets `title` into the index by **naming the key** in their own facet
+policy, with no change here. Door C's index label is the concept path, so for
+those senders the title travels as a facet or not at all.
+
+### D-C3. An unrenderable value drops the FACET, never the CONCEPT — and is reported
+
+The policy refuses a value carrying its own separator or joiner rather than
+escaping it. At Door B that refuses the document, and rightly: the value is one
+we derived and the operator can fix the source.
+
+At Door C it must not. This door judges no shape and refuses no sender on form —
+that is what the module docstring already commits to — so refusing a merge over
+a semicolon in someone else's frontmatter is precisely the failure it names.
+Dropping it silently is the other failure: the sender made a claim our index
+does not show. So the facet is dropped, the concept merges verbatim, and the
+drop is reported per concept and key in `ImportResult.unrendered_facets` — an
+advisory over the merged set, like `unverified_references`, never a fifth
+bucket.
+
+### D-C4. A faceted entry is refreshed, because it is the first one that can go stale
+
+Found by measurement during the work, not predicted. A flat entry carries a
+label and a target, both stable, so it can never disagree with the file it
+points at, and `link_in_index`'s early return on a present target is exactly
+right. An entry carrying the concept's **facts** can disagree.
+
+The path is reachable: the collision gate refuses an updated concept outright,
+so the operator's only route to accept an update is to remove the merged file —
+which the refusal message itself instructs — and import again. Measured after
+doing so: the concept file said `gjeldende` while the index still said `utkast`.
+
+An index that contradicts the bundle it indexes is worse than one that says
+nothing, because the consumer reads the index and stops. A faceted entry for a
+present target is now re-rendered in place, keyed on the policy's entry pattern
+and the parsed target — never on a substring, so a curated line that merely
+mentions the target survives verbatim, line ending included. With `facets=None`
+none of this runs, so every unfaceted caller emits the bytes it always did.
+
+This is the one addition beyond the order's three edits. It is here because it
+is a divergence *this change introduced*, and shipping a measured file/index
+contradiction would have been worse than the scope it costs.
+
+## Verification
+
+Suite **695 → 709**; `ruff check`, `ruff format --check`, `mypy --strict` clean.
+`import_bundle`'s `profile` is keyword-only and defaults to `DEFAULT`, so the
+171 branch bases built through this door emit the bytes they always did — pinned
+by a test that feeds the DEFAULT path the same frontmatter that *would* produce
+a facet tail under a faceted profile.
+
+### Mutation testing, and the harness that lied first
+
+11 mutations of the new code, plus a permanent **negative control** (a
+behaviour-preserving local rename that must SURVIVE).
+
+The control earned its place on its first run. The harness invoked
+`pytest --timeout=120`; `pytest-timeout` is not installed, so every run exited
+non-zero and every mutant was reported **killed — 11 of 11**. The number was
+entirely false: zero mutations had actually been evaluated. The control is what
+exposed it, by being reported killed when it provably could not be. A clean
+sweep is worthless until the harness has been shown able to report a survivor.
+
+The harness now asserts the suite passes on **unmutated** source before a single
+"killed" is believed, and asserts per mutant that the anchor matched exactly
+once and that the mutation landed on disk.
+
+Honest result: **9 of 11 killed.** Both survivors are equivalent mutants, not
+gaps:
+
+- **M1** (`if not value` → `if value is None`) — `FacetPolicy.render` already
+  filters falsy values (`profiles.py:423`), so the guard in `_project_facets` is
+  redundant with the policy's own. The observable property is now pinned
+  directly anyway: a key the sender declared empty produces no facet.
+- **M8** (drop the `changed` guard before writing) — writes byte-identical
+  content.
+
+**M10** — reverting the `profile` threading in `import_filename` — survived the
+first honest run, and that one *was* a gap: every profile shipped today reuses
+`DEFAULT.paths`, so no existing test could tell a threaded profile from a
+hard-coded `DEFAULT`. The property was held by coincidence. A profile owning its
+own `import_prefix` now states it directly, and M10 is killed.
+
+## What was not achieved
+
+1. **`title` is not in `STRUCTURED_V1`'s facet set.** Widening it would move
+   Door B's index bytes, so it stays a caller's key to name. For a sender whose
+   concept paths are UUIDs this is the difference between a title in the index
+   and none — flagged to vegnormal-okf rather than decided here.
+2. **Door C's index is an append plus a per-target refresh, not a projection of
+   the whole bundle** (contrast D-S4). A concept removed from the bundle by hand
+   leaves its entry behind. Reprojection would mean enumerating the directory,
+   which no profile has asked this door to do.
+3. **Parent-prefix resolution is still deferred**, unchanged and for the
+   unchanged reason: "unique prefix match" must be defined against a real
+   corpus. `vegnormal-okf` has that corpus; the count is theirs to take.
+4. **The before/after number is not ours to produce.** Whether the faceted index
+   is worth its cost on a real corpus is answered by re-running the same arm
+   with the same scoring, on their data.