chore: Add raycast grid annotation infrastructure for an upcoming screenshot capability

[hatayama]

Summary

• Ports the raycast grid annotation infrastructure from main (RaycastGridAnnotator, RaycastHitClusterer, RaycastLayerMaskResolver, RaycastSampleOutlineBuilder) so a later PR can wire it into the screenshot tool.
• Extracts a reusable UiRaycastHelper.RaycastContext so repeated UI-occlusion raycasts (up to 1600 per screenshot capture) can reuse cached Canvas/EventSystem state instead of rescanning every sample.

User Impact

• No visible behavior change yet — these classes are not called by any tool in this PR. They exist so the upcoming screenshot raycast-grid feature can build on them.
• Existing UI raycast behavior (mouse-click simulation, drag targeting, input replay) is unchanged; RaycastUI now delegates to RaycastContext internally but keeps identical hit-testing semantics.

Changes

• Added RaycastGridAnnotator/RaycastHitClusterer/RaycastLayerMaskResolver/RaycastSampleOutlineBuilder under FirstPartyTools/Screenshot/RaycastAnnotation/ (only the screenshot tool uses these on main, so no new shared Common module was introduced).
• Added RaycastGridPointInfo, RaycastLayerSummaryInfo, RaycastOutlineSegment to ToolContracts, and extended UIElementInfo with Layer/Components/RaycastOutlineSegments.
• Extracted CollectCanvasRaycastSources/RaycastCanvasSpaceFromSources from the existing UiRaycastHelper.RaycastCanvasSpace, and built a new RaycastContext on top of them. Kept this repo's existing (more correct) IsRaycastCandidate filtering instead of copying main's older logic, to avoid two divergent UI hit-testing implementations.
• Ported RaycastGridAnnotatorTests.cs (30 tests) with a what-comment on each test.

Verification

• uloop compile: 0 errors, 0 warnings
• RaycastGridAnnotatorTests (30/30 green)
• MouseUiDragTargetResolverTests + MouseUiPointerTargetResolverTests (EditMode, 13/13 green)
• SimulateMouseUiTests + SimulateMouseUiInputSystemTests + InputReplayVerificationE2ETests (PlayMode, 34/34 + 2/2 green) — confirms the UiRaycastHelper refactor has no behavior regression for existing callers

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 0b278d2c-154a-44c4-8ca6-59d71b8b7bfe

📥 Commits

Reviewing files that changed from the base of the PR and between 5352e00 and 5e15af7.

⛔ Files ignored due to path filters (10)

• Assets/Tests/Editor/RaycastGridAnnotatorTests.cs.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Screenshot/RaycastAnnotation.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Screenshot/RaycastAnnotation/RaycastGridAnnotator.cs.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Screenshot/RaycastAnnotation/RaycastHitClusterer.cs.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Screenshot/RaycastAnnotation/RaycastLayerMaskResolver.cs.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Screenshot/RaycastAnnotation/RaycastSampleOutlineBuilder.cs.meta is excluded by none and included by none
• Packages/src/Editor/FirstPartyTools/Screenshot/UnityCLILoop.FirstPartyTools.Screenshot.Editor.asmdef is excluded by none and included by none
• Packages/src/Editor/ToolContracts/RaycastGridPointInfo.cs.meta is excluded by none and included by none
• Packages/src/Editor/ToolContracts/RaycastLayerSummaryInfo.cs.meta is excluded by none and included by none
• Packages/src/Editor/ToolContracts/RaycastOutlineSegment.cs.meta is excluded by none and included by none

📒 Files selected for processing (11)

• Assets/Tests/Editor/RaycastGridAnnotatorTests.cs
• Packages/src/Editor/FirstPartyTools/Common/GameView/AssemblyInfo.cs
• Packages/src/Editor/FirstPartyTools/Common/MouseUi/UiRaycastHelper.cs
• Packages/src/Editor/FirstPartyTools/Screenshot/RaycastAnnotation/RaycastGridAnnotator.cs
• Packages/src/Editor/FirstPartyTools/Screenshot/RaycastAnnotation/RaycastHitClusterer.cs
• Packages/src/Editor/FirstPartyTools/Screenshot/RaycastAnnotation/RaycastLayerMaskResolver.cs
• Packages/src/Editor/FirstPartyTools/Screenshot/RaycastAnnotation/RaycastSampleOutlineBuilder.cs
• Packages/src/Editor/ToolContracts/RaycastGridPointInfo.cs
• Packages/src/Editor/ToolContracts/RaycastLayerSummaryInfo.cs
• Packages/src/Editor/ToolContracts/RaycastOutlineSegment.cs
• Packages/src/Editor/ToolContracts/UIElementInfo.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/raycast-grid-annotation

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