feat(theme): add dynamic XP appearance

[doodlewind]

Summary

• add a System UI theme boundary that owns both paint classes and chrome hit-test metrics
• add a Sheru-informed Windows XP Luna appearance with three-stop plastic gradients, gel caption controls, taskbar, Start menu, application surfaces, and white control glyphs
• expose live switching through Start > Settings and Cmd+Shift+T while preserving every window client rectangle, including child CompositorSurface viewports
• keep the Pocket System manifest, AppSupervisor, native host, and AppInstance plans theme-neutral
• pin PocketJS to merged #327 for generic via-* gradient stops

Validation

• bun run check (50 unit tests, 4 System UI sim journeys, 3 wrap-op tests)
• bun run build:web && bun run test:web (real headless Chrome; Hero AppInstance started and composited)
• bun run build
• bun run macos -- --quit-after 5 (native release host launched; 5 ticks, 4 frames)
• bun run capture with visual inspection of docs/xp-theme.png
• git diff --check

Geometry contract

The same ChromeMetrics drive rendering and hit testing. Theme switches reframe normal, minimum, and maximized/restore geometry around a stable client rectangle; tests pin the XP 21px caption controls, 2px gaps, 30px taskbar, and client-size round trip.

Assets

The XP caption glyphs are original code-generated SVGs. No Sheru files or third-party Windows assets are copied.