test(errors): reunite the segmentation codes with the conformance suite

The registry file says one test per code and that this file IS the
conformance suite. During the 1-to-N voyage the byte-stability pin over
tests/ was the verification mechanism, so editing this file was
forbidden and six new codes were covered in the new segmentation
modules instead. That was right then. The end-to-end gate run released
the pin, so the convention applies again -- a conformance suite split
across two files stops being one quietly, which is why this was carried
as an obligation rather than a preference.

All seven segmentation codes now have a test here, including
segmentation_plan_unmatched from this session. The behavioural tests in
test_segmentation.py and test_segmented_inbox.py stay where they are:
they exercise the parser and the door, a different question from
whether every documented code has a raise site.

Measured after: 49 documented codes, 48 covered in this file. The one
gap, source_reference_unquotable, predates this work and is tested in
tests/test_okf_v0_2_profile.py:351 -- reported, not silently closed.

Also records both defects and this migration as closed in the decision
record's known-gaps list.

Co-Authored-By: Claude <claude-opus-5>

docs/plan/concept-granularity.md

@@ -118,23 +118,43 @@ hidden: a consumer meets both mechanisms.
 
 ## Known gaps, stated rather than left to be discovered
 
-1. **`derived` marks a declared title as inferred.** A segmented concept's
-   `title` is DECLARED by the adjudicator, but structure derivation runs over
-   the segment body and adds `title` to `derived`. The marker is therefore
-   wrong in the one direction this repo cares about — a consumer that distrusts
-   derived fields will distrust a fact the producer stated. The golden pins
-   today's behaviour; the fix belongs with the code that composes the two.
-2. **A plan matching no dropped file is silent.** A typo'd `source_sha256`
-   segments nothing and reports nothing, which is the silent skip this library
-   refuses elsewhere.
+1. ~~**`derived` marks a declared title as inferred.**~~ **Closed 2026-09-01.**
+   A segmented concept's `title` is DECLARED by the adjudicator, but structure
+   derivation ran over the segment body and added `title` to `derived`. The
+   marker pointed the wrong way in the one direction this repo cares about — a
+   consumer that distrusts derived fields would have distrusted a fact the
+   producer stated. `title` is now dropped from `derived` when a segment is
+   present, and only then. The fix is scoped to `title` alone and pinned that
+   way: an inferred `number` still marks itself on a segment, because nothing
+   about segmentation makes a derived document number declared, and blunting
+   the marker would be the opposite defect. The `SEGMENTED_V1` golden moved
+   with it; the four others were measured byte-identical to baseline.
+2. ~~**A plan matching no dropped file is silent.**~~ **Closed 2026-09-01.**
+   A typo'd `source_sha256` segmented nothing and reported nothing, which is
+   the silent skip this library refuses elsewhere. `process_inbox` now refuses
+   fail-fast with `segmentation_plan_unmatched` before any disk mutation. Two
+   sharper questions had to be settled to make the refusal correct rather than
+   merely loud. It asks whether a covering plan was **found**, not whether
+   every file was examined, so an unreadable drop cannot suppress it. And
+   coverage is recorded at **selection**, not after path validation: the first
+   cut asked the later question, and a plan whose hash matched but whose entry
+   paths were then refused looked identical to a plan that matched nothing —
+   the operator would have been sent to check a hash that was already right.
+   An existing collision test caught it; the case now has its own test,
+   verified red against the earlier form.
 3. **`status: deprecated` has no producer.** `status` is named as a facet key
    and nothing writes it, so a deprecated parent cannot carry one.
 4. **An unchanged round still rewrites.** `write_bytes` writes
    unconditionally; the bytes are identical, so no contract is broken, but
    "an unchanged round writes nothing" is not true today.
-5. **Six new error codes live in `tests/test_segmentation.py`, not
-   `tests/test_error_codes.py`.** The baseline pin forbade editing the
-   registry file. Migrate once the pin is released.
+5. ~~**Six new error codes live in `tests/test_segmentation.py`, not
+   `tests/test_error_codes.py`.**~~ **Closed 2026-09-01.** The baseline pin
+   forbade editing the registry file while it was the voyage's verification
+   mechanism. The gate run above released it, so the registry convention — one
+   test per code, this file IS the conformance suite — applies again, and the
+   codes were migrated. A conformance suite split across two files stops being
+   one quietly, which is why this was carried as an obligation rather than a
+   preference.
 
 ## The end-to-end gate run, and its one failure