perf: skip deep SSTable validation on fast import

[zxch3n]

Summary

• Skip eager SSTable block decoding/key-order validation when importing through the fast path.
• Keep per-block checksum verification enabled for both strict and fast imports.
• Update SSTable tests to cover strict validation and the fast-path behavior.

Validation

• cargo test -p loro-kv-store
• cargo test -p loro-internal import_atomicity
• rustfmt --check crates/kv-store/src/sstable.rs
• git diff --check
• cargo bench -p loro-internal --features test_utils --bench encode -- B4_decode_snapshot: [234.50 us 239.03 us 247.50 us]

Note

• cargo fmt --all --check still reports pre-existing unrelated formatting differences outside this change.

[github-actions]

WASM Size Report

• Original size: 3039.61 KB
• Gzipped size: 1002.84 KB
• Brotli size: 703.73 KB