Fix March 2026 production panic regressions

[zxch3n]

Summary

• fix stale DAG causal iteration after lazy node splits so export/import no longer drop same-peer ranges
• fix richtext IdToCursor fragment-tail updates and zero-width update handling
• add a list import_batch() regression that exercises the same split-tracking path seen in production
• recover document-owned locks after poisoning instead of cascading PoisonError panics
• add unpoisoned lock helpers across core internals so ordinary Mutex users follow the same recovery rule

Verification

• cargo test -p loro-internal stale_iterator_state_repairs_missing_same_peer_continuation -- --nocapture
• cargo test -p loro-internal larger_node_only_advances_consumed_slice -- --nocapture
• cargo test -p loro-internal repeated_tail_splits_keep_id_to_cursor_consistent -- --nocapture
• cargo test -p loro-internal zero_width_small_update_keeps_insert_set_non_empty -- --nocapture
• cargo test -p loro-internal large_update_can_replace_the_tail_range -- --nocapture
• cargo test -p loro-internal list_import_batch_stays_consistent_after_repeated_tail_splits -- --nocapture
• cargo test -p loro-internal poison -- --nocapture
• cargo check -p loro-internal

Notes

• the active ImVersionVector export panic still was not reproducible in this checkout; no speculative fix was included for that path
• the FFI-side unpoison work lives in /Users/zxch3n/Code/loro-ffi on branch codex/unpoison-locks and is not part of this PR

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