feat(world): crash-safe saves (verify-readback + .bak snapshot)

[proggeramlug]

Editor production-readiness item 5.2 (editor PLAN.md Part 5): saveWorld/savePrefab now write <path>.tmp, read it back and byte-compare (catches the historical zero-byte-write-reports-success failure class), snapshot the previous file to <path>.bak, then write the real file — a crash at any point leaves a good copy on disk. True atomic replace needs a rename FFI (future).

Verified by the editor's new tools/ui-smoke.ps1 interaction test.

https://claude.ai/code/session_01PmL9WgNMkAgvpYSHEZga8K

Summary by CodeRabbit

• Bug Fixes
  • Improved world and prefab save reliability by verifying saved data before finalizing files.
  • Existing save files are backed up before being replaced.
  • Save failures now return clearer error results while preserving validation behavior.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 3b67ad3c-4e09-4fec-a7ae-efc27b1b4d9c

📥 Commits

Reviewing files that changed from the base of the PR and between 57de189 and 4cbbb96.

📒 Files selected for processing (1)

• src/world/saver.ts

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📝 Walkthrough

Walkthrough

src/world/saver.ts adds verified temporary writes, optional .bak snapshots, and final-file replacement. saveWorld and savePrefab now use this flow and return prefixed errors on failure, while validation behavior remains unchanged.

Changes

Crash-safe world and prefab saves

Layer / File(s)	Summary
Verified write and backup flow src/world/saver.ts	safeWrite writes to a temporary file, verifies read-back content, optionally creates a .bak snapshot, and writes verified content to the final path.
World and prefab integration src/world/saver.ts	saveWorld and savePrefab call safeWrite and prefix returned errors with their operation names.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant SaveOperation
  participant safeWrite
  participant Filesystem
  SaveOperation->>safeWrite: JSON path and content
  safeWrite->>Filesystem: Write temporary file
  safeWrite->>Filesystem: Read back and compare content
  safeWrite->>Filesystem: Snapshot existing file to .bak
  safeWrite->>Filesystem: Write verified content to final path
  safeWrite-->>SaveOperation: Success or error

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: crash-safe world and prefab saves with readback verification and backups.
Description check	✅ Passed	The description covers the change and verification, though it does not fully follow the template’s checklist and reviewer-notes sections.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/safe-saves

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