docs: 1.4.7 release notes, vendor-global decision, architecture pages

[lesnik512]

Documentation close-out after the two architecture reviews. No code changes.

1. Release notes — planning/releases/1.4.7.md

Drafts 1.4.7 (patch). Headline: the case-insensitive vendor matching fix (#18) — a mixed-case configured vendor previously never matched and silently fell back to v1.0. Also lists the internal refactors (#17, #21). No API changes, so downstream needs no action. The tag/publish is yours to cut — this only stages the note (versioning is tag-driven).

2. Decision — planning/decisions/2026-07-04-vendor-media-type-stays-process-global.md

Records review #2's candidate B (vendor as process-global class state) as rejected, closing that review's loop so it isn't re-suggested. Rationale: instance-scoping collides with the response-class mechanism kept by the ClassProperty decision, and one-vendor-per-process is an accepted, documented constraint. Includes a revisit trigger (needing two different vendors in one process).

3. Architecture pages — architecture/

Seeds the living-truth home, which until now held only its README:

• glossary.md — the ubiquitous language (Vendor media type, Version, Versioned route, …).
• accept-header-version-negotiation.md — the parse → match → respond flow across accept.py / router.py.
• openapi-schema-versioning.md — _iter_openapi_routes + _collapse_versioned_paths and why only request bodies are rewritten.

Both pages cross-reference the three decision records for the deliberate constraints.

just check-planning → planning: OK; just lint-ci clean.

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