chore: simplify external Scene change handling

[hatayama]

Summary

• Give each external Scene change responsibility explicit production file ownership.
• Use one shared file-state comparison contract for compile and Prefab focus-return decisions.

User Impact

• There is no intended user-visible behavior change.
• External Scene reload policy, focus-return handling, snapshot persistence, messages, and cleanup ordering remain unchanged.

Changes

• Keep ExternalSceneChangeResolver in its existing matching file.
• Move ExternalSceneChangeTracker, ExternalAssetSnapshotSessionStore, and ExternalAssetFocusReturnService unchanged into matching files.
• Add ExternalAssetFileStateComparer and replace the two byte-identical private comparison methods with direct calls.
• Preserve the existing (Exists, LastWriteTimeUtc, Length) tuple shape instead of introducing a broader fingerprint data-model migration.

Verification

• Passed the existing resolver fixture before editing (16/16).
• Compared all four moved class bodies against the base revision; each class body was byte-identical after the file split.
• Confirmed the new comparer tests fail before implementation with four CS0103 errors, then pass after implementation (4/4).
• Passed the combined comparer and resolver tests (20/20).
• Passed the related comparer, resolver, compile use case, architecture, and static facade guard fixtures (124/124).
• Compiled the Unity package with 0 errors and 0 warnings.

Compatibility

• SessionState keys, serialized snapshot JSON fields, event subscriptions, mutable static state ownership, visibility, messages, and comparison short-circuit order are unchanged.
• No IPC request or response shape changed, so the protocol version remains unchanged.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: bf225af6-490c-4170-91bf-b4621ee84d13

📥 Commits

Reviewing files that changed from the base of the PR and between 37b9f5d and 1f530a6.

⛔ Files ignored due to path filters (5)

• Assets/Tests/Editor/ExternalAssetFileStateComparerTests.cs.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Compile/ExternalAssetFileStateComparer.cs.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Compile/ExternalAssetFocusReturnService.cs.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Compile/ExternalAssetSnapshotSessionStore.cs.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Compile/ExternalSceneChangeTracker.cs.meta is excluded by none and included by none

📒 Files selected for processing (6)

• Assets/Tests/Editor/ExternalAssetFileStateComparerTests.cs
• Packages/src/Editor/FirstPartyTools/Compile/ExternalAssetFileStateComparer.cs
• Packages/src/Editor/FirstPartyTools/Compile/ExternalAssetFocusReturnService.cs
• Packages/src/Editor/FirstPartyTools/Compile/ExternalAssetSnapshotSessionStore.cs
• Packages/src/Editor/FirstPartyTools/Compile/ExternalSceneChangeResolver.cs
• Packages/src/Editor/FirstPartyTools/Compile/ExternalSceneChangeTracker.cs

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📝 Walkthrough

Walkthrough

Introduces a shared ExternalAssetFileStateComparer for fingerprint equality checks, new ExternalAssetFocusReturnService and ExternalAssetSnapshotSessionStore utilities, and a new ExternalSceneChangeTracker that persists scene/Prefab fingerprints across domain reloads and resolves external changes on compile and editor focus return. Updates ExternalSceneChangeResolver to use the shared comparer.

Changes

External asset change tracking

Layer / File(s)	Summary
Shared file-state comparer and tests Packages/src/Editor/FirstPartyTools/Compile/ExternalAssetFileStateComparer.cs, Assets/Tests/Editor/ExternalAssetFileStateComparerTests.cs, Packages/src/Editor/FirstPartyTools/Compile/ExternalSceneChangeResolver.cs	Adds HasSameFileState comparing Exists/LastWriteTimeUtc/Length, unit tests covering match and mismatch cases, and updates the resolver to delegate to it instead of a removed local helper.
Focus-return service and snapshot session store Packages/src/Editor/FirstPartyTools/Compile/ExternalAssetFocusReturnService.cs, Packages/src/Editor/FirstPartyTools/Compile/ExternalAssetSnapshotSessionStore.cs	Adds a delegate-driven service that holds/restores auto-refresh state based on editor focus, and a JSON-based session store for serializing/deserializing snapshot dictionaries.
Tracker initialization and compile resolution Packages/src/Editor/FirstPartyTools/Compile/ExternalSceneChangeTracker.cs	Adds Initialize() to restore snapshots and register scene/Prefab Stage/focus event handlers, and ResolveForCompile() to resolve external Scene changes and persist updated snapshots.
Focus-return resolution and prefab reopen Packages/src/Editor/FirstPartyTools/Compile/ExternalSceneChangeTracker.cs	Saves dirty/missing scenes and Prefab Stage, resolves external Scene/Prefab changes, reimports and reopens changed prefabs with instance-aware reopen context, and logs failures.
Fingerprinting and persistence helpers Packages/src/Editor/FirstPartyTools/Compile/ExternalSceneChangeTracker.cs	Adds snapshot recording, trackability checks, fingerprint reading, scene setup reload, resolver construction, SessionState persistence, and path normalization/display/logging utilities.

Estimated code review effort: 4 (Complex) | ~60 minutes

Possibly related PRs

• hatayama/unity-cli-loop#1447: Introduces ExternalSceneChangeResolverTests covering ExternalAssetFileStateComparer, ExternalAssetFocusReturnService, and ExternalAssetSnapshotSessionStore that are directly used by this PR.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title concisely matches the main change: simplifying external Scene change handling.
Description check	✅ Passed	The description is clearly related to the code changes and summarizes the split and shared comparer.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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 refactor/hatayama/split-external-scene-change-resolver

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