Make memory accounting lock-free

[alexy]

Implements docs/lock-free.md as designed; that file now records the outcome.

ExecutionContext holds accounted bytes and their high-water mark as atomics and admits a memory charge through the same compare-exchange loop as a work charge, so a byte limit stays exact under concurrent charges. Releases are one fetch_sub. Only the cancellation wakers stay behind the mutex.

Why: the reference Cypher executor charges the logical bytes of every copied value, so a streaming query charges memory once or more per element. The lock was about 11% of the profile of the full-path reduce query.

Behaviour that changes

1. usage() reads its three figures in sequence, not as one snapshot. It reports max(peak, live), so peak_bytes >= live_bytes for every reader; read after execution for exact totals.
2. The peak can trail a charge for an instant; it never misses a completed charge.
3. A poisoned lock no longer fails a memory charge. Only waker registration can report poisoning.

Tests

tests/contracts/memory.rs: sixteen threads racing for a limit that fits exactly fifty chunks admit exactly fifty, over fifty rounds; cross-thread drops return live_bytes to zero; the peak is a true maximum, unmoved by refusals, never observed below live by a racing reader; overflow is a budget failure. Not model-checked with loom. 1,297 tests pass across grust-core, grust-cypher, grust-memory, grust-turso, grust-algorithms, grust-algorithm-procedures and grust-datafusion.

Measured (one laptop, release, whole-process wall time, best of seven)

query	nodes	mutex	lock-free
reduce fold	1,024	301 ms	267 ms
reduce fold	4,096	4,389 ms	3,815 ms
UNWIND aggregate	4,096	862 ms	859 ms
direct kernel	4,096	108 ms	107 ms

The lock-free figure was measured before and after the mutex figure and reproduced. Paths that charge memory rarely do not move, so the regression the plan guarded against did not appear here. The paired benchmark-harness run has not happened and these figures are not a substitute for it.

🤖 Generated with Claude Code