Add ARD v0.91 draft: define the ARD entry, decouple from ai-catalog

[rvguha]

What this is

Adds the v0.91 draft (spec/ard-v0.91-draft.md) alongside the published v0.9 (spec/ard.md is untouched). The draft is annotated with editorial footnotes and ◆ breadcrumbs marking every change from v0.9; an editorial box at the top explains the conventions. All annotations are review aids and are to be removed before publication.

Headline change: ARD defines the ARD entry

§4 establishes that the ARD entry is a distinct object from a catalog entry, with a different obligation — a catalog accommodates whatever a publisher lists; an ARD entry guarantees the signals a search service needs. The normative formulation:

Every ARD entry is a well-formed catalog entry, but not every catalog entry is an ARD entry.

Because the definitions are separate, each spec's conformance is self-contained: a future reduction of any catalog core cannot silently change what ARD requires.

Normative changes (flagged in place)

• representativeQueries is now REQUIRED for an ARD entry (§4.2). It is the signal the semantic index is built from; an entry lacking it cannot be found by search. A listing without it remains a valid catalog entry — just not an ARD entry.
• Well-known path / link relation are now ard-named: /.well-known/ard.json, rel="ard" (§5.1). The former ai-catalog names are honoured as aliases (Compatibility note), so nothing published today breaks.

New artifact: ARD owns its entry schema

• spec/schemas/ard-entry.schema.json — authoritative for the ARD entry, does not derive from any catalog schema. Structure: entryFields (open, additionalProperties: true so namespaced terms stay valid) → ardEntry (full, requires the discovery terms + value-or-reference) and ardEntryProjection (search-result shape, only identifier required, so results may omit representativeQueries).
• spec/schemas/ard.openapi.yaml — search/list result $refs repointed to ardEntryProjection.

Structural (no behaviour change)

• Description layer restated on JSON-LD with a default namespace + @context extension seam.
• Identity & Trust folded into the entry model (§4.5).
• Discovery reorganized as the umbrella over the search API and federation.

Validation

• ard-entry.schema.json compiles; full entry requires representativeQueries, projection does not, namespaced extra terms accepted.
• All three §4 example entries validate against the required terms + value-or-reference.
• Footnotes: 24 markers, 24 definitions, no orphans.

Reviewer notes / open items

• The default namespace still supplies term definitions (ai-catalog as default); this PR decouples ownership, not the default vocabulary. Remaining ai-catalog mentions in the publishable body: one Compatibility note (unavoidable — you can't alias a name without naming it).
• conformance/bin/conformance-test still loads the catalog schema and validates whole manifests; it needs a separate change to iterate entries[] against ard-entry.schema.json. Deliberately not included here.
• The generated .docx is intentionally excluded (build artifact).

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