026-references-provenance-derived-at

References provenance: optional derived_at timestamp

Adds an optional `derived_at` field to the `Provenance` struct carried on a `references` edge: `{ kind, ref, derived_at? }`. `derived_at` is a generic, optional ISO-8601 timestamp recording when the reference was derived, preserved verbatim with no format validation in the type (an adopter that wants format enforcement adds a lint). The field is additive and optional, so a corpus that declares none is unaffected and `deny_unknown_fields` is preserved (the field becomes known, not a hole in the schema). The registry format gains an emittable field, so `REGISTRY_SCHEMA_VERSION` bumps an additive minor 1.0.0 -> 1.1.0 (the permissive shard schema file is unchanged; the bump only restamps the `specVersion` carried by each committed registry shard, leaving every `shardHash` and `record` body byte-identical, because a shard hash is taken over the `spec.md` source bytes). Filed off the OAP spec-217 engine-swap, whose decomposition synthesizer (OAP spec 165) emits this shape because OAP spec 161 FR-007 requires `provenance.derived_at`; the published library rejected it at parse with `V-002: unknown field derived_at`, producing zero shards. This is the second small, broadly-useful additive concession from the 217 swap (the first was the spec-027 symbol-resolution feature gate, shipped as 0.7.0).