feat(steering): cross-rank applied-action checksum in dynamic status

[RhizoNymph]

Failure mode

Sync capture consumers run identically and independently on every TP rank in lock-step, with zero communication. Each rank applies the returned steering actions via SteeringModelRunnerMixin._apply_steering_actions. The contract is enforced only by convention: if one rank hits a local fault (consumer OOM, a swallowed exception at the observer boundary in _run_sync_consumers), its dyn_id allocation and steering tables silently desync from its siblings forever, corrupting output with no error surfaced anywhere.

Detector

An always-on, cheap rolling checksum of applied steering actions per worker:

• Worker (steering_model_runner_mixin.py): _apply_steering_actions folds every action that is actually applied (rejected actions excluded) into self._steering_action_checksum (u64). The fold is zlib.crc32 over a compact, PYTHONHASHSEED-free digest of the action content (class, target req_id/config_hash/dyn_id, hook/layer, source, and a bit-exact shape+CRC of any vector/probe payload) mixed splitmix-style in application order, plus a per-drain-batch ordinal so "same actions, different step" differs. Digests are bit-exact (not norms) because actions are host-side numpy built from rank-identical inputs — strictly stronger and never legitimately divergent across ranks. O(applied actions); zero cost on idle steps. The rollback of a failed declarative override is itself folded. get_dynamic_steering_status exposes action_checksum (hex) + action_count (picklable primitives).
• Router (_merge.py + api_router.py): GET /v1/steering/dynamic compares checksums across workers via check_action_determinism. A mismatch does not 500 — the response carries determinism: {consistent: false, checksums: {...}} and a rate-limited server-side ERROR fires; on match determinism: {consistent: true, action_count: N}.
• Reuses steering_update_accepted (new thin wrapper over the existing _validate_update) so the batched vector-update path folds exactly the applied set with no duplicated validation logic.

Topology scoping / granularity caveat

Comparison is scoped within each PP stage (grouped by pp_rank), mirroring deep_merge_status: TP ranks in a stage own identical layers and must match, while PP stages own disjoint layers and may legitimately differ. Sync-consumer-originated actions only exist at pipeline_parallel_size == 1 (enforced in vllm/v1/capture/registry.py), where this reduces to an all-workers comparison — exactly right. Granularity is poll-time: a desync is detected on the next status poll, so the checksum bounds (does not prevent) corruption; pair with periodic polling.

Docs

New docs/design/dynamic_steering.md §6.1 documents the detector and its poll-time granularity.

Expected sibling-PR conflicts

• chore/steering-row-owner touches steering_manager.py and the mixin's scale/monitor apply methods (different regions of the same file).
• Docs conflicts possible in docs/design/dynamic_steering.md with chore/steering-trust-hardening.