Feat: add Unity Magic Exam Hall playable prototype

[SilverSupplier]

Summary

• Add a Unity 6000.3.14f1 Magic Exam Hall project with a 5-floor world-casting gameplay loop.
• Implement right-click world drawing, base family recognition, overlay operators, seal stacks, floor goals, hazards, ending report, and session logging.
• Add Unity EditMode/PlayMode tests plus README/.gitignore updates for Unity execution and generated state.

Verification

• node scripts/validate-doc-state.mjs
• Unity scene builder: MagicExamHall.Editor.MagicExamHallSceneBuilder.BuildAll
• Unity EditMode tests: 19 passed
• Unity PlayMode tests: 4 passed

Notes

• Unity generated folders and test/log outputs are ignored and not included in this PR.
• This PR is based directly on origin/main so it stays separate from docs PR Docs: add Magic Exam Hall game design roadmap #10.

Closes #23

[SilverSupplier]

Feedback pass added in bd529da.

Implemented:

• base failure escalation wired into Magic Note: first/second/third failures now show reason hint, checklist, then stronger family-specific guidance
• success after assisted attempts now keeps assisted feedback state in runtime logging
• overlay failure copy now explains size/anchor/dependency next actions, including martial_axis requiring void_cut first
• PlayMode coverage added for Magic Note escalation and assisted success feedback

Verification:

• Unity scene builder: passed
• Unity EditMode: 19 passed / 0 failed
• Unity PlayMode: 6 passed / 0 failed

Refs #14, #38, #39, #40.

[SilverSupplier]

World casting timing pass added in d5f82c6.

Implemented:

• multi-stroke buffer default moved from 0.8s to 1.05s for steadier wind/three-stroke input
• base seal lifetime default moved from 7.5s to 11s so players have more room to attach overlays
• scene builder and runtime now apply the same playable timing defaults, even when older serialized scene values exist
• stroke visual lifetime now tracks the longer buffer window
• Unity README controls updated from 0.8s to about 1 second

Verification:

• Unity EditMode: 20 passed / 0 failed
• Unity PlayMode: 6 passed / 0 failed
• docs validation via bundled node: passed

Refs #13, #35.