fix(core): reject root-level primitives in initialState

[lodystage]

Summary

• LoroDoc only stores container types (Map/List/MovableList/Text/Tree) at the document root, but Mirror previously accepted root-level primitives in initialState (e.g. { version: 0 }) and silently kept them in memory while never persisting them to the doc — making mirror.getState() drift from doc.toJSON().
• Reject primitives at the type level via a new RootInitialValue constraint on initialState, and at runtime with a clear error that points users to wrap the value in a root LoroMap (e.g. a meta map).
• null/undefined at the root are still treated as "absent" and silently skipped.

Repro (now fails fast)

const doc = new LoroDoc();
new Mirror({ doc, initialState: { version: 0, notes: [] } });
// TS error on `version`; throws at runtime if `as any` is used.

Test plan

• Added 4 runtime tests in packages/core/tests/mirror.test.ts covering: bare number, bare boolean, primitive under a (mistyped) schema, and null/undefined skip-behavior.
• Added a type-level test using @ts-expect-error to lock in the TypeScript rejection of root-level number/boolean.
• All 456 tests pass; 0 type errors.

🤖 Generated with Claude Code