feat: Phase 8 — ts-migrate muya/lib (148 JS→TS) + eliminate all explicit any

Migrated all 148 JavaScript files in src/muya/lib/ to TypeScript using
ts-migrate, then systematically eliminated all explicit `any` annotations
across every subdirectory using 5 parallel agents.

Typecheck: 1,212 errors → 0  |  noExplicitAny in muya/lib: 0
Unit tests: 522/522 pass  |  @ts-expect-error baseline: 242

Key type additions to src/muya/lib/types/index.ts:
- Token, LineCursor, SelectionInfo, SelectionFormatsResult
- IDragInfo, ICellSelectInfo, IEventCenter
- ~80 missing IContentState method signatures

Biome noExplicitAny set to "warn" (was "off").
Justified any kept with biome-ignore comments (~46 instances):
mixin `this: any`, untyped third-party libs (Prism, TurndownService,
element-resize-detector, mermaid), dynamic tokenizer Token interface.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>