[upstream-sync] refactor(state_store): remove ReadTxn from API; standalone reads per call (upstream #2010)

[github-actions]

Upstream Change Summary

Type: architectural-change
Difficulty: Medium
Recommendation: Adopt

Continuation of the state_store refactor series beyond what issue #168 tracked (upstream #1981–#1991). PR #2010 removes ReadTxn from the public AppStore API entirely. Read methods now come in two flavors:

• *_in_txn(wtxn, ...) — for reads inside a write transaction (sees uncommitted writes)
• xxx(...) — standalone reads that open their own internal snapshot

Storage::read_txn, read_txn_for_inspect, Environment::read_txn, and the ReadTxn/AnyTxn types are removed from the public surface. AppStore now carries a clone of the parent Env to support standalone reads. The MDB_READERS_FULL two-phase retry moves from Storage into AppStore standalone reads. spawn_iter_stable_paths_with_node_type is migrated from std::thread::spawn to tokio::task::spawn_blocking (the LMDB cursor is !Send so it still cannot cross .await, but the expression is idiomatic).

Upstream References

• PR: refactor(state_store): remove ReadTxn from API; standalone reads per call cocoindex-io/cocoindex#2010
• Upstream repo: https://github.com/cocoindex-io/cocoindex
• Related recoco issue: [upstream-sync] refactor(state_store): LMDB isolation, async I/O, and fn-memo cache series (upstream #1981–#1991) #168 (covers upstream #1981–#1991)

Relevant Upstream Files / Areas

• rust/cocoindex/src/state_store/ — AppStore, Storage, ReadTxn, AnyTxn, storage.rs
• rust/cocoindex/src/engine/ — all call sites migrated from env.read_txn() pattern

Recoco Considerations

• Affected files: crates/recoco-core/src/state_store/, crates/recoco-core/src/engine/
• Pre-condition: Verify the current state of recoco's state_store after issue [upstream-sync] refactor(state_store): LMDB isolation, async I/O, and fn-memo cache series (upstream #1981–#1991) #168 is resolved before applying this — the diff depends on what was already ported
• Call sites pattern change: let mut rtxn = env.read_txn().await?; app_store.read_x(&mut rtxn, ...) → app_store.read_x(...) (standalone); in-write-txn callers use app_store.read_x_in_txn(wtxn, ...)
• No Python code to exclude; no feature-gating required
• The tokio::task::spawn_blocking migration for spawn_iter_stable_paths_with_node_type is a correctness improvement for async contexts

Integration Notes

This is a direct continuation of #168's state_store series. Block this issue on #168 being resolved first. The core change is an API simplification that hides transaction management from callers — conceptually straightforward but touches many call sites. The !Send cursor constraint means the spawn_blocking migration needs special care; the upstream uses RoPrefix which wraps a raw *mut MDB_cursor.