Fix checkpoint publication and recovery boundaries

[GCdePaula]

Closes the review fixes for stack #40, stacked on #43. Enforces the checkpoint and recovery boundaries needed to avoid publishing invalid finalized state, replaying already-accounted inputs, or deleting registered snapshots.

• Refuse finalized snapshot/state exports and accepted /history receipts after persisted canonical divergence, including a matching batch followed by divergent acceptance in the same L1 block.
• Require the trusted checkpoint block B to be at or before the fixed recovery stop C before replay or baseline publication. A lagging L1 view returns retryable exit 20. Document the exporter's pending-direct eligibility obligation: A < B alone is insufficient.
• Treat missing or structurally corrupt referenced snapshot artifacts as terminal, including failures discovered during streaming; preserve operational I/O error classification.
• Preserve registered snapshots across relative/absolute paths, symlinks, and mount aliases by comparing filesystem identities. Inspect every retained reference before deleting orphans.
• Bound SDK snapshot response-header waits while allowing the body to outlive that timeout. Strengthen regressions for the strict recovery-prefix cutoff, checkpoint publication failures, crash leases, and cold-replica state; retain harness logs on CI failure.

Validation at 209a3b1:

• macOS host suite: 760 passed, zero failed, one existing ignored doctest. Workspace check, strict Clippy, formatting, and Markdown links passed; the new firmlink regression fails against the old path comparison and passes with the fix.
• Both push CI and PR CI completed successfully, including Rust, canonical guest, rollups E2E, and watchdog image checks. The PR run passed all 49 rollups scenarios; watchdog Lua reported 3 passed, 1 opt-in scenario skipped.

Detailed validation records the test scope and remaining application-specific recovery-export work. The earlier intermittent C-host WebSocket reset remains an open investigation; the added CI logs support diagnosing a recurrence.

[GCdePaula]

Superseded by #45.