perf: eliminate queue/thread overhead in Python; remove redundant storage Mutex and use parking_lot in Rust

[Copilot]

The Rust-backed dictsqlite_v2 and the pure-Python dictsqlite both had structural bottlenecks preventing them from reaching their potential throughput.

Python (dictsqlite/main.py)

• Replaced queue.Queue + daemon worker thread with threading.RLock — every read previously required allocating a queue.Queue, two thread context-switches, and a blocking get(); now it's a single uncontested lock acquisition.
• WAL mode on by default with synchronous=NORMAL, 64 MB cache_size, temp_store=MEMORY, and 128 MB mmap_size — no longer requires the caller to opt in.
• operation_queue kept as a _SyncQueue no-op stub for backward compatibility (join() is now a no-op since all ops are synchronous).
• Fixed a deadlock bug in _process_queue_conflict_resolver that recursively called _process_queue().
• Errors from corrupt/read-only databases now propagate immediately rather than being silently swallowed by the background thread.

Rust (dictsqlite_v2)

Remove redundant Mutex around StorageEngine (lib.rs)

StorageEngine is internally thread-safe via its r2d2 connection pool and DashMap. Wrapping it in an additional Mutex was pure overhead on every read and write:

// Before — mutex acquired on every storage access
storage: Arc<Mutex<Option<StorageEngine>>>

// After — direct Arc, no lock needed
storage: Option<Arc<StorageEngine>>

Fix unconditional LRU mutex acquisition in get() / set() (lib.rs)

The WriteThrough mode caused access_tracker (a Mutex<LruCache>) to be locked on every hot-tier read, even when the cache was nowhere near its eviction threshold. The condition now only tracks when approaching capacity, consistent with the behaviour of other persist modes.

Switch std::sync::Mutex → parking_lot::Mutex (lib.rs, async_ops.rs)

parking_lot was already a declared dependency but std::sync::Mutex was being used instead. parking_lot::Mutex has lower overhead and removes the need for unwrap() on lock acquisition.

[github-actions]

Dependency Review

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