[openapi-mcp S4] Projection domain module: index, canonical identity, ladder, error views

[rickylabs]

Part of #1126

Scope

Projection domain module in packages/mcp (pure, no runtime deps): operation index; canonical-identity law — resolve to exactly one spec operation (exact dotted id, else exact METHOD path), ambiguity refuses with candidates, case-insensitive/substring matching is suggestion-display only (S-2); description ladder with deterministic rung 3 = humanized operationId (S-22); schema views (request/response/errors/all) with error views derived from each operation's actual declared responses — common-envelope compaction only when detected present (S-19).

Blocked by S2's verdict artifact (proofs/P2-verdict.md).

Design source: RFC #1123 (rfc.md §2.4) + design/canonical/03-projection-and-naming.md (rev 2).

Acceptance

• gate: per-rung ladder fixtures incl. a real generated no-summary spec proving which rung fires — fixture/test evidence
• gate: ambiguity-refusal fixtures (case-variant ids; input matching >1 operation) — fixture/test evidence
• gate: errors-view fixture on the no-database template proving no hallucinated envelope — fixture/test evidence