fix(#670): self-healing recovery for installer-migration checksum drift (#675)

Editing the body of an already-released installer migration drifts its computed
checksum (it hashes plan.toString()). The integrity guard then hard-aborted
every prior install on upgrade with "applied migration checksum changed" — a
100% reproducible blocker (v1.3.0, all platforms).

Already-applied migrations are filtered out of `pending` and never re-run, so
a drifted checksum is functionally inert. ADR-0008 anticipates checksum-mismatch
state as something the install-state layer must handle gracefully (plan -> apply
-> recover/report), not abort on.

This supersedes the published-checksum allowlist merged in #674 (per-release
maintenance debt — every historical checksum hand-pinned, still throws for any
unregistered value) with a general, self-healing recovery:

- Replace the throwing guard with non-fatal `collectAppliedChecksumDrift`,
  surfaced on `plan.checksumDrift`.
- Reconcile drifted stored checksums durably on the next state write
  (`reconcileDriftedChecksums`), idempotently (no perpetual writes).
- Relocate the "shipped migration bodies are immutable" rule to a CI baseline
  test that locks every shipped migration's checksum and fails on body drift —
  where #615 should have been caught, instead of blocking users.

Removes #674's legacyChecksums field, per-migration checksum pins,
published-checksums.json fixture, and compat test.

Fixes #670

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/fierce-lynx-howl.md

@@ -1,5 +0,0 @@
----
-type: Fixed
-pr: 674
----
-Accept published installer migration checksums so Windows users upgrading from 1.2.0 to 1.3.0 do not hit checksum drift failures.

.changeset/humble-ibex-jump.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 675
+---
+**`npx @opengsd/gsd-core` upgrades no longer abort with "applied migration checksum changed"** — an already-applied installer migration whose recorded checksum drifted (e.g. a shipped body was edited) is now detected and reconciled automatically on the next install, instead of hard-failing the upgrade. Replaces the published-checksum allowlist with general self-healing recovery plus a CI baseline lock.