feat: clean up runtime-core to single authoritative protocol implementation

- Remove all legacy/duplicate src files: canonical.ts, encoding.ts,
  errors.ts, normalize.ts, schema-client.ts, types.ts (all used wrong
  protocol or imported ajv which is not a dep)
- Remove root-level duplicate TS files (old pre-src layout)
- Remove stale tests/runtime-core.test.mjs (imported removed API)
- Remove instructional APPLY.md and DELETIONS.sh (already applied)
- Fix crypto.ts: use sign(null,...)/verify(null,...) for Ed25519 —
  createSign("Ed25519") is invalid in Node.js crypto API
- Fix package.json test script: --require tsx/esm → --import tsx/esm
  (ESM packages require --import, not --require)
- Fix test/crypto.test.ts: static import of createHash, async callback

All 41 tests pass (canonicalize, crypto, receipt round-trip).

https://claude.ai/code/session_01GJsQ9fnXzuYDuhtBuByKeW

Author: claude