Fix Core 0.7 input boundaries before acceptance (0.8.5)

[schchit]

Changes

• Align current event-schema extension identifiers and full-string digest matching with Core/Agent validators.
• Validate decoded current JWS header values without re-encoding the signed protected bytes.
• Preserve explicitly selected legacy parsing and all historical signed artifacts.
• Add real-signature HTTP archival/acceptance regressions, corrected same-identity retries, and noncanonical valid UTF-8 positive controls.
• Prepare software 0.8.5; protocol remains Core 0.7 / wire 1.

Evidence

Local targeted API tests: 31 passed. Cross-repository test against actual Core/API/Agent code: 25 cases, previously 14 inconsistent cases, now 0; full hosted checks must pass before merge. Existing acceptance state is not reset or migrated.

The patch was applied against source 0ed259f and the resulting tree was checked against 0fd679048c093bbcc985b0e5692d7e8050257352. The temporary patch-application files are not included in this PR.

No frozen draft, historical signature, credential configuration, or Prooftask change.