fix(compat): reject unfingerprinted continuation writes

[bokelley]

Summary

• reject new continuation rows that omit an issuance fingerprint after replay-fence migration
• reject attempts to downgrade a fingerprinted continuation to a legacy NULL fingerprint
• preserve existing pre-fingerprint rows while preventing older workers from minting fresh authorizations

Validation

• uv run pytest -q tests/test_purchase_continuation.py (120 passed)
• uv run pytest -q (6,876 passed, 40 skipped, 9 deselected, 1 xfailed)
• scoped pre-commit checks (Black, Ruff, mypy, adopter fixtures, Bandit, whitespace)
• protocol and security review: release-ready, no findings

[aao-secretariat]

Ladon verdict: Approve

Approve — defense-in-depth hardening on the SQLite continuation ledger with no blocking or medium findings.

Checked:

• No no-auto-approve team match; gated_paths is false; review_decision REVIEW_REQUIRED (informational only — gated_paths=false, so row 2 does not fire).
• high_risk=true only because src/adcp/compat/sqlite_continuation_store.py is (modified) — no medium-or-higher finding on it, so it is presumed safe (row 5 requires a medium finding, none exist).
• Two new BEFORE triggers reject NULL-fingerprint INSERTs and fingerprint downgrades on modern rows; happy path unchanged (INSERT always supplies a fingerprint; continuation UPDATE only touches claimed_operation_id). Legacy NULL rows remain readable/updatable via OLD IS NOT NULL guard. Trigger names added to _REPLAY_FENCE_TRIGGER_NAMES and _REPLAY_FENCE_SCHEMA for atomic/idempotent migration. Docs match behavior; tests cover both guards and the legacy-row replay path.

No rows 1–8 fired → row 9 approve.