fix: add long peer fuzz and richtext diff catchup

[zxch3n]

Summary

• Add long_peer_fuzz, a deterministic single-thread multi-peer fuzz runner that does not use cargo fuzz.
• Write failing artifacts with seed/config/history/state snapshots and a minimized replay when possible.
• Document usage, seed control, artifact layout, and long-run examples in crates/fuzz/README.md.
• Fix richtext checkout diff so full-history tracking starts from an exact empty tracker and catches up missing text ops before computing checkout diffs.
• Keep shallow-history richtext tracking on the existing unknown-base path because older ops may be unavailable.

Root cause

The long peer fuzz found a state-only roundtrip mismatch after checkout. One peer state rendered as u32[18283][18265]28[128][3], while importing the state-only export rendered u32[18283][18265]28[128][3]12.

The richtext checkout diff tracker used an anonymous unknown span as the LCA base. In cross-branch checkout, the target can retain old text that the source deleted. Since that anonymous span has no real id/materialized text, the diff can treat text as already represented and omit the insert/delete needed to reach the target state.

Fix

For full-history docs, richtext diff tracking now uses an exact empty tracker and catches up CRDT text ops from the covered version to the oplog version before applying checkout diffs. covered_vv avoids repeating the full catchup when later diffs can extend from already-covered state.

DiffCalculatorTrait::start_tracking now receives ContainerIdx, so richtext can catch up the correct container. Shallow-history tracking still uses the unknown base path.

Validation

• cargo check -p loro-internal
• cargo check -p fuzz --bin long_peer_fuzz
• cargo test -p fuzz --test long_peer_repro state_only_round_trip_with_shallow_deps_does_not_abort -- --nocapture
• cargo test -p loro state_only_shallow_since_and_updates_till_cover_export_boundaries
• cargo test -p loro-internal shallow_dep_ensure_vv_for_uses_shallow_baseline
• cargo test -p loro shallow_snapshot_test
• cargo run -p fuzz --release --bin long_peer_fuzz -- --seed 20260629 --peers 12 --ops 10000 --sync-barrier-every 5000 --check-every 20000 --no-minimize --artifact-dir long_peer_fuzz_artifacts/tmp-10k-after-catchup
• cargo run -p fuzz --release --bin long_peer_fuzz -- --seed 20260629 --peers 12 --duration-secs 60 --sync-barrier-every 5000 --check-every 20000 --no-minimize --artifact-dir long_peer_fuzz_artifacts/one-minute-final-diff-fix

Next: run the same long peer fuzz for 8 hours before marking ready.