[Feature]: Add a redraw heatmap overlay for repaint debugging

[forketyfork]

Title: [Feature]: Add a redraw heatmap overlay for repaint debugging

Labels: feature, major

Status Quo

Architect has metrics and logging, but there is no spatial debugging tool that shows what part of the UI or terminal content was rerendered on a given frame. When typing feels slow or rendering seems wasteful, the only current options are code inspection and log-based tracing. The overlay system in src/ui/ is already the right place for a developer-facing visualization layer, but no redraw-debug component exists today.

Objectives

Give developers a visual tool to understand redraw behavior in real time. When enabled, Architect should highlight recently rerendered regions with a fading color overlay so it is obvious whether the app is repainting only what changed or repainting much more than necessary.

User Flow

Trigger: The developer enables redraw-debug mode.

1. The developer turns on redraw visualization through a debug setting or shortcut.
2. As terminal content or UI changes are rendered, Architect records the affected region and shows a fading overlay.
3. The developer types, scrolls, opens overlays, or switches modes and watches which regions flash.

Result: Redraw behavior is easy to inspect visually without reading logs.

Scope

In scope:

• Add a developer-facing redraw-visualization mode with fading colored highlights.
• Visualize at least terminal-content rerenders, with room to distinguish content rerenders from whole-frame redraws later.

Out of scope:

• Exact GPU-driver-level damage reporting.
• Shipping the overlay as always-on user-facing UI.

Implementation Plan

Affected Modules

• src/render/renderer.zig: emit redraw region information when terminal content is rerendered.
• src/ui/components/: add a new overlay component that renders fading highlight rectangles.
• src/ui/root.zig: register and render the new debug component.
• src/app/runtime.zig: wire toggle behavior and pass per-frame redraw data through the existing UI and render flow.
• src/config.zig: optional config flag if the mode should be user-configurable.
• docs/configuration.md: document the debug toggle if it is exposed in config.

Tasks

1. Define a redraw-debug data model for recording rerendered regions per frame. src/render/renderer.zig or a small shared module
2. Emit redraw events for terminal content rerenders in grid and full-screen paths. src/render/renderer.zig
3. Add a UI overlay component that renders fading highlight rectangles on top of the scene. src/ui/components/, src/ui/root.zig
4. Add a debug toggle path through runtime and optional config or shortcut plumbing. src/app/runtime.zig, src/config.zig
5. Write tests for region lifecycle, fade timing, toggle behavior, and near-zero overhead when disabled.
6. Update docs/configuration.md if a config toggle is added.
7. Update docs/ARCHITECTURE.md to document the redraw-debug overlay and data flow.

New Dependencies

None

Acceptance Criteria

• Developers can enable and disable redraw visualization without changing production behavior.
• Recently rerendered regions are highlighted with a visible fade-out effect.
• The overlay distinguishes meaningful redraw activity well enough to debug repaint behavior.
• Tests cover the happy path and documented edge cases.
• docs/configuration.md is updated if config is added.
• docs/ARCHITECTURE.md is updated.
• Pre-commit hooks pass.