fix(avm): avoid data race on shared interaction selector writes (port #24456 to v5-next)

[fcarreiro]

Ports #24456 to v5-next.

What was wrong

The interactions tracegen phase runs every lookup/permutation job concurrently: AvmTraceGenHelper::fill_trace_interactions concatenates all builders' jobs and dispatches them with parallel_for. Lookups whose fine-grained destination selector is a shared column (DST_SELECTOR != outer_dst_selector) can resolve to the same dst_row from different jobs, so multiple threads write the same (selector, row) cell at once.

The previous code did a guarded read-modify-write on that shared cell:

if (DST_SELECTOR != outer_dst_selector && trace.get(DST_SELECTOR, dst_row) != 1) {
    trace.set(DST_SELECTOR, dst_row, 1);
}

Both the get and the non-atomic 32-byte set race against the other threads' writes to the same cell — a data race, i.e. undefined behavior. In practice it produced the right value (every writer stores 1), but it is still UB.

The fix

Add an opt-in use_atomic_limbs flag to TraceContainer::set. When set, the field's four 64-bit limbs are written with relaxed atomic stores — four plain movq on x86-64, no lock and no libatomic call. (A whole-field std::atomic_ref<FF> is not an option on the hot path: at 32 bytes it exceeds the hardware lock-free width and falls back to a locked libatomic call.)

The shared selector write now uses it and drops the guard read:

if (DST_SELECTOR != outer_dst_selector) {
    trace.set(DST_SELECTOR, dst_row, 1, /*use_atomic_limbs=*/true);
}

The write is an unconditional, idempotent atomic store of 1. Because every concurrent writer stores the same value, per-limb atomicity is sufficient — no torn value is possible — so this is data-race-free without the cost of whole-field atomicity. The default (non-atomic) set hot path is untouched and keeps its sparse-column "zero = absent" fast path.

Performance (this PR vs baseline)

Mega bulk AVM tx, full proving, HARDWARE_CONCURRENCY=16. Tracegen stage timings, median of runs (baseline n=3, this PR n=6):

Stage	baseline	this PR
tracegen traces	1,536 ms	1,537 ms
tracegen interactions	350 ms	329 ms
tracegen all	1,917 ms	1,936 ms

No measurable cost — the safety fix is free. The per-limb atomic only fires on the shared selector writes in the interactions stage; the traces stage is byte-for-byte the same hot path as baseline.

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/812cb13379eef261�812cb13379eef2618;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/single-node/proving/proof_fails.parallel.test.ts "does not allow submitting proof after epoch end" (117s) (code: 0) group:e2e-p2p-epoch-flakes