Document edge-served 304 metadata contract

[AMBRA7592]

Summary

• distinguish origin-generated 200 metadata headers from edge-served 304 responses
• make the non-immutable supersession index the authoritative status and successor-discovery surface
• explain why the pre-coexistence production build returns 404 for an inactive v0.1.3 ID while the coexistence preview returns 200 with inactive status
• tell release operators to assert only 304 status and an empty body
• require a one-time cache-busting metadata probe to show x-vercel-cache: MISS, making origin delivery observable

Why

The pre-activation staging audit observed that Vercel can answer a conditional request from its edge cache with a compliant zero-byte 304 while omitting application-defined x-trialdiff-* metadata headers. The application still emits those headers at origin, but clients and release operators must not treat them as a conditional-revalidation contract.

Impact

Documentation only. No application code, migration, frozen package, manifest, implementation hash, database state, deployment, activation, or publication changes.

Verification

• python3 -m unittest discover -s tests: 95 passed
• alpha and v0.1.1/v0.1.2/v0.1.3 package validators: passed
• root, calibration, and all event-class package manifests: verified
• git diff --check: passed
• runtime and frozen-package diff against main: empty