perf(tooling): add an animation cost profiler and document the harness

Adds `bun run profile:animation`: it serves an isolated fixture and measures
each animation variant directly, so comparing techniques takes seconds instead
of an application rebuild plus a streamed response.

The result is unambiguous and does not vary with element count, measured from 1
to 32: transform, opacity and filter cost zero extra style recalculations, while
the individual rotate property, background-position, border-color and box-shadow
each recalculate style 60 times a second, and geometry properties add layout on
top. Notably `rotate: 360deg` is not a cheap synonym for
`transform: rotate(360deg)`, and will-change, wrapper elements, containment and
stepped timing do not make a non-composited property cheap.

`scripts/perf/DOCUMENTATION.md` documents all four capture commands, how to
stand up a production build to measure against, how to read the artifacts, the
validity guarantees the scripts enforce, and the methodology rules, so this can
be handed to an agent as the entry point for measuring performance. It is linked
from the root guide's documentation anchors.

The theme skill gains an animation contract carrying the measured table, and the
performance skill points at the tooling documentation.

AGENTS.md

@@ -65,6 +65,7 @@ High-value anchors:
 - Sync: `packages/ui/src/sync/DOCUMENTATION.md`
 - Stores: `packages/ui/src/stores/DOCUMENTATION.md`
 - CLI: `packages/web/bin/lib/DOCUMENTATION.md`
+- Performance measurement tooling: `scripts/perf/DOCUMENTATION.md`
 - VS Code runtime: `packages/vscode/src/DOCUMENTATION.md`
 - Electron: `packages/electron/README.md`
 - Mobile: `packages/mobile/README.md`