Add stress lock contention mode to stress testing

[markolazic01]

stress: add lock-contention mode with XMSS verify skip (#825)

Closes #825

Adds a ZEAM_STRESS_SKIP_VERIFY env variable that skips XMSS signature verification (when set to 1 or true) in the gossip-flood worker's onBlock path.

Solution details

Went with option (a) (described inside #825) due to it having less maintenance surface further on.

Changes

• chain.zig: add skipVerify: bool to CachedProcessedBlockInfo, gate the verifySignatures / verifySignaturesParallel call on it. Defaults to false — no production caller sets it.
• stress.zig: add readEnvBool helper, read ZEAM_STRESS_SKIP_VERIFY in StressConfig, thread it through StressCtx into the gossip-flood worker's onBlock call.
• build.zig: add stress-locks-quick build step (30s, skip verify), wired into zig build test.

Results

Run with 60s and 6 gossip threads

Metric	Default	Skip verify
gossip-flood ops	962	853
borrow-readers ops	22,550	55,059
total ops/sec	756	1,370

Gossip-flood throughput drops despite less per-iteration work, confirming lock contention is now the bottleneck rather than XMSS verify. Total system lock traffic nearly doubles.

Safety

skipVerify defaults to false in CachedProcessedBlockInfo and is only set by the stress harness. Pre-import still verifies signatures to build trusted chain state.

[markolazic01]

Resolved a conflict

[markolazic01]

it seems workflow fail is unrelated to the changes in this PR