feat: damage tracking + debug overlay interfaces (ADR-065, ADR-066)

[kolkov]

Summary

• Damage tracking interfaces (ADR-065) — multi-renderer damage aggregation for shared GPU surfaces
• Pluggable debug overlay system (ADR-066) — GTK4 Inspector-inspired overlay architecture

New types

• DamageReporter interface (2 methods, frozen for v1.0+)
• DamageCategory enum (6 categories)
• DamageReason struct
• DamageSourceSnapshot struct
• DamageOverlayRenderer interface
• DamageOverlayInfo struct
• DebugOverlay interface (2 methods, frozen for v1.0+)
• DebugOverlayContext struct

Design references

• Chromium cc/DamageTracker — damage aggregation pattern
• GTK4 GtkInspectorOverlay — pluggable overlay system
• Flutter debugRepaintRainbow — debug overlay activation

Test plan

• go test ./... — passes
• golangci-lint run — 0 issues
• Cross-platform build (linux, darwin, windows)
• Feature audit: all interfaces fully implemented, no stubs

[codecov]

Codecov Report

❌ Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
damage.go	0.00%	16 Missing ⚠️

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