docs(plan): record the structure-derivation decisions, mutations and cost

Six decisions, each with the failure it prevents: confidence carried in the
format, certainty stated per field, supersession never inferred from one
document, the index as a projection rather than an append, facets as a new
profile, and Door B's ownership stamp left deliberately open.

Six findings reported as findings rather than as gaps, per the order: the cost
of closing the metadata gap is 3.3x-6.4x the flat index and whether that keeps
the OKF arm's cost advantage cannot be answered from this repo -- it needs
their bake-off re-run on their corpus; a reference to a parent-level number
does not resolve to a child, and the fix needs a real corpus before 'unique'
can be defended; OKF_V0_2's verified/status/stale_after are still expressible
and unwritten for the reason already recorded.

Every number carries its denominator, and the supersedes 0/55 carries a
known-positive control (50/55 on a corpus that has shared numbers) proving the
query can find what it reported missing.

README.md

@@ -163,8 +163,22 @@ instead of refusing. `DEFAULT` remains OKF v0.1 on every axis upstream owns.
 | `DEFAULT` | commons' ingest-spec layer (OKF v0.1 semantics) | stable |
 | `STRICT_V1` | a consumer's ratified v0.1 contract | stable |
 | `OKF_V0_2` | OKF v0.2 | **provisional**, pre-release only |
+| `STRUCTURED_V1` | `DEFAULT` plus a faceted, derived index | stable |
 | `OKF_LATEST` | alias for the latest version supported as *stable* | currently `DEFAULT` |
 
+`STRUCTURED_V1` is `DEFAULT` in every respect but the index. Under it, Door B
+derives each dropped document's title, number, hierarchy and cross-references,
+writes them into the concept's own frontmatter, and carries them into the index
+entry — so a consumer can reason over the bundle rather than only look things
+up in it. Every inferred field is named in a `derived` list, because an
+unmarked heuristic is worse than no heuristic: the consumer cannot know when to
+doubt it. A pointer to a document not dropped yet is rendered `N200?` rather
+than omitted, since a bundle is built up over several drops and an absence that
+leaves no trace is the dangerous kind. Carrying the metadata costs index
+characters — roughly 3x to 6x the flat index, depending on how many facets the
+profile names — and the facet key set is the dial. Design record and
+measurements: [`docs/plan/structure-derivation.md`](docs/plan/structure-derivation.md).
+
 `OKF_V0_2` ships first as a pre-release to a named pilot set and may change on
 their feedback without a deprecation cycle. Pin the versioned constant rather
 than `OKF_LATEST` unless you have explicitly opted into tracking; `OKF_LATEST`