feat(core): add withCid ($cid) support for maps and tree node.data

[zxch3n]

• Add CID_KEY constant ($cid) to identify container id in mirrored state
• Types: extend InferType so LoroMap/LoroMapRecord with { withCid: true }
  infer { ...; $cid: string }; add tree node variant when nodeSchema has withCid
• Mirror snapshot: register roots before snapshot; add containerToStateJson and
  buildRootStateSnapshot to normalize trees and inject $cid; use in
  initialize + checkStateConsistency
• Events: extend applyEventBatchToState with optional options object
  { getContainerById, containerToJson, nodeDataWithCid, getNodeDataCid }
  (backward compatible with function arg); stamp $cid on tree “create” during
  event application; container inserts (map/list) go through containerToJson
• Writes: ignore $cid in diffMap and map initialization/updates (read-only)
• Tests: add packages/core/tests/cid.test.ts covering initial snapshot,
  event inserts, tree creates, and diff ignoring $cid (use doc.commit where needed)
• Docs: update root README and packages/core/README.md to document withCid,
  $cid semantics, and examples (e.g., list idSelector via x => x.$cid)
• Plan: update plan.md with completed progress and changelog

Notes:

• $cid exists only in mirrored state; it is never written to Loro
• No behavior change for maps without { withCid: true }