fix: reduce loro-js snapshot memory

[zxch3n]

Summary

• keep imported latest-state SSTables encoded and hydrate containers one block at a time
• retain snapshot history as an encoded base plus a small materialized overlay for local and imported changes
• rewrite only dirty state blocks during snapshot export and remove redundant output-buffer copies
• add regression coverage, a real-document memory benchmark, performance notes, and a patch changeset

Why

Importing the 11.4 MB test document eagerly decoded 115,147 containers and materialized the full 423,797-operation history. A local edit then duplicated more history/state structures, and snapshot export allocated another set of encoded buffers. This amplified the document to roughly 703 MiB heap after import, over 860 MiB after one edit, and about 1.66 GiB peak RSS during snapshot export.

The new representation keeps the immutable snapshot bytes as the base, materializes only accessed or dirty state, and appends new history to an overlay until an API genuinely needs arbitrary historical traversal.

Impact

On the supplied 11,387,982-byte test document:

• snapshot import: about 0.90 s
• local commit: about 1.9 ms
• full update export: about 6.6 ms
• snapshot export: about 57 ms
• process baseline after loading input: 70.92 MiB RSS
• process peak after snapshot export: 160.70 MiB RSS
• incremental peak: 89.78 MiB
• peak used JS heap: 8.58 MiB

The ordinary fully materialized snapshot path did not regress. After three warmups, two 15-sample B4 A/B runs measured 110.5/107.0 ms medians at the parent revision and 107.7/107.7 ms on this branch. Both emitted identical 309,780-byte snapshots.

Validation

• pnpm --dir loro-js check
  • format check
  • lint
  • typecheck
  • 229 tests across 16 files
  • package build
• git diff --check
• real-document snapshot/update version-vector and operation-count validation
• real-document snapshot state comparison across all 115,147 entries
• local and remote edit snapshot round trips

[github-actions]

WASM Size Report

• Original size: 3053.22 KB
• Gzipped size: 1009.02 KB
• Brotli size: 707.57 KB