feat: add hybrid input architecture modules (draft)

Add three new modules for the hybrid input architecture that combines
PTE's behavior system with ProseMirror-style parse-and-diff as a
fallback for platforms where preventDefault is unavailable (Android).

New modules:
- dom-parser.ts: Parses DOM back into PT blocks using data-slate-*
  attributes. Uses ELEMENT_TO_NODE WeakMap for unchanged nodes (fast
  path) and DOM walking for browser-mutated content (slow path).
  Handles text blocks, void objects, inline objects, and containers.
- dom-change-detector.ts: Diffs two PT block arrays to detect what
  changed. Aligns blocks by _key, detects splits/merges from content
  shape, and does character-level text diffing for inline changes.
  39 unit tests covering all change types.
- dom-change-to-behavior-event.ts: Maps detected changes to PTE
  behavior events (insert.text, insert.break, delete.backward, etc.)
  that can be dispatched through the existing behavior system.

These modules are standalone — no integration with the input pipeline
yet. This is Phase 1 of the hybrid architecture (see design doc).

Author: christianhg