Add best-effort layout tree source hints

[static-var]

Summary

• add nullable best-effort source hint fields to experimental layoutTree nodes:
  • sourceName
  • sourceFile
  • sourceLine
  • sourceHintKind
• preserve existing componentHint and layout tree structure
• enrich layout tree nodes in the forked Robolectric renderer using Compose tooling data when available
• correlate tooling groups to runtime LayoutNodes by identity plus bounded app-source/preorder fallback
• keep source hint extraction optional and non-fatal
• document best-effort behavior and CMP fallback caveat in snapshot schema docs

Notes

• source hints are additive and nullable; schemaVersion remains 2
• sourceHintKind indicates confidence/source, e.g. tooling-nearest-app-ancestor or preview-entrypoint-fallback
• CMP Android-target sample currently falls back to preview-entrypoint hints when tooling data is unavailable

Verification

• build-brief ./gradlew spotlessCheck detekt :plugin:test :preview-scanner:test
• ANDROID_HOME=$HOME/Library/Android/sdk build-brief ./gradlew -p samples/android-compose-app :app:captureComposePreviews --configuration-cache
• ANDROID_HOME=$HOME/Library/Android/sdk build-brief ./gradlew -p samples/cmp-compose-app :composeApp:captureComposePreviews --configuration-cache

Android sample source-hint check

Inspected generated snapshots and found 70 useful non-fallback tooling hints, for example:

layout-2 Surface LoginPreview.kt 36 tooling-nearest-app-ancestor
layout-3 Column LoginPreview.kt 37 tooling-nearest-app-ancestor
layout-4 Text LoginPreview.kt 41 tooling-nearest-app-ancestor
layout-5 Spacer LoginPreview.kt 42 tooling-nearest-app-ancestor
layout-6 Box LoginPreview.kt 43 tooling-nearest-app-ancestor

Review

• Implementation delegated to subagent
• Multiple review/fix cycles addressed fallback-only output and framework/internal source names
• Final review subagent approved with no findings