Document stale cached supersession metadata

[AMBRA7592]

Summary

• document that immutable cached 200 responses can retain pre-activation supersession metadata even while canonical bytes and ETags remain exact
• define record-status and predecessor/successor headers as advisory, point-in-time metadata
• retain the non-immutable supersession index as the authoritative source for current status and successor discovery
• require an origin-confirmed x-vercel-cache: MISS response for release-time header assertions

Context

The v0.1.3 production cutover audit observed stale metadata on edge-cached 200 responses:

• cached v0.1.2: current, without successor
• origin v0.1.2: superseded, with successor
• cached v0.1.3: inactive
• origin v0.1.3: current

Canonical bodies and ETags remained byte-exact. PR #6 had already merged before this finding arrived, so this is a documentation-only follow-up.

Scope

Only RELEASING.md, frontend/README.md, and frontend/AGENTS.md change. No code, schema, manifest, frozen package, hash, tag, deployment, or publication artifact changes.

Verification

• 95 passed, 2 subtests passed
• all frozen-package validators passed
• schema validation passed for 40 + 100 + 97 + 97 records
• all root and package manifests passed
• offline trialdiff verify: verified=PASS failures=0
• git diff --check passed