fix(tooltip): dismiss plain tooltips on leave

[Banegasn]

Summary

• Dismiss plain tooltips immediately when their trigger is left.
• Retain the rich-tooltip grace period for pointer travel to interactive content.
• Synchronize the plain-tooltip browser contract with the immediate dismissal lifecycle.

Validation

• m3-tooltip tests: Chromium, Firefox, and WebKit — 21 tests per engine.
• Lint and typecheck.
• Coverage ratchets, tokens, build, production audit, package smoke, Svelte smoke, and license verification.

The full pnpm test lane is blocked by an unrelated baseline WebKit failure in m3-menu: its real Tab key test expects #after but receives document.body. The focused menu run reproduces it on unchanged main code.