perf: Skip updating registry if signature has not changed

[FrederikBolding]

Since verifying and updating the registry can be expensive on slower devices, we can store the current signature and only re-verify when the signature changes.

---

Note

Stores and persists the registry signature to skip verification and database updates when unchanged, updating controller logic and tests.

• Snaps Registry Controller (json.ts):
  • Add signature to SnapsRegistryState and controller metadata (persisted, logged; not exposed to UI).
  • Update flow: fetch registry.json and signature.json, parse signature; if signature matches state, skip verification/update and only refresh lastUpdated/databaseUnavailable.
  • On change, verify using parsed SignatureStruct and persist new database and signature.
  • Adjust #verifySignature to accept SignatureStruct (using @metamask/superstruct Infer).
• Tests (json.test.ts):
  • Add test ensuring update is skipped when signature unchanged; update metadata snapshots to include signature.
• Dependencies:
  • Add @metamask/superstruct.

^{Written by Cursor Bugbot for commit 8ff96dd. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (24b87d3) to head (8ff96dd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3779   +/-   ##
=======================================
  Coverage   98.33%   98.33%           
=======================================
  Files         420      420           
  Lines       12019    12032   +13     
  Branches     1872     1874    +2     
=======================================
+ Hits        11819    11832   +13     
  Misses        200      200           

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