fix(core): avoid double mirror startup traversal

[zxch3n]

Summary

• avoid building the root mirror snapshot twice during construction
• register nested map/list containers while walking the startup snapshot instead of doing a separate nested scan first
• keep tree startup registration on the existing nested scan path

Validation

• pnpm --filter loro-mirror test
• pnpm check
• git diff --check

Trace note

The Lody trace shows the long startup task split between initializeContainers and buildRootStateSnapshot. This change removes that duplicate map/list traversal. Local Node + loro-crdt/web benchmarks on the provided updates.json still stay in the tens of milliseconds, so this fixes the clear duplicate work but does not by itself explain the full multi-second Chrome trace.