feat(mem-wal): add ShardWriter::put_no_wait

[hamersaw]

What

Adds ShardWriter::put_no_wait, a variant of put that performs the visible in-memory insert and triggers the durable WAL flush, then returns the BatchDurableWatcher without awaiting it. A thin wrapper restores put's behavior (await the watcher), so existing callers are unchanged.

put_memtable is split into:

• put_memtable_no_wait — the in-memory critical section (insert under state_lock + track_batch_for_wal + flush triggers) followed by trigger_flush, returning (WriteResult, Option<BatchDurableWatcher>).
• put_memtable — calls the above, then watcher.wait().

BatchDurableWatcher and WriteResult are re-exported from mem_wal.

Why

Lets an external caller hold its own serialization lock across only the in-memory read-merge-insert critical section and await durability after releasing it, so concurrent durable flushes still coalesce. The in-memory insert stays guarded by the writer's state_lock, so BatchStore's single-writer invariant holds regardless of the external lock — state_lock is intentionally not skipped.

This is the lance-side primitive for sophon's WAL partial-column-update path (read fresh tier → merge → insert under a per-bucket lock, durability awaited outside it).

Tests

test_put_no_wait_durable_visible_then_durable (row visible before durability, watcher resolves) and test_put_no_wait_non_durable_returns_no_watcher.

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/mem_wal/write.rs	97.67%	0 Missing and 2 partials ⚠️

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