M12-T02: Extract shared wal frame codec after retire_queue lands

[skel84]

Summary

Extract the shared WAL frame codec only after the first micro-extraction lands cleanly.

Scope

• factor the shared WAL frame encoding/decoding discipline into one internal runtime boundary
• keep engine-local payload decoding outside the shared layer
• preserve bounded, fail-closed framing behavior

Acceptance

• the shared layer owns frame structure and checksum discipline only
• payload semantics stay engine-local
• no WAL behavior regresses in any engine

Canonical docs

• docs/runtime-extraction-roadmap.md
• docs/reservation-runtime-seam-evaluation.md

Blocked by

• M12: Extract the first internal shared runtime substrate from the three-engine family #120