Record a consent decision

Records a visitor's consent decision without an event, and returns the consent now in force for them. Applied immediately rather than queued: a withdrawal has to bind the next delivery. It is not billed as an event, and it is never forwarded to a destination — it decides what may be. Resolving identity inline means a call whose identifiers span two persons merges them, taking the workspace-scoped merge lock on the request path. Measured at ~6ms against ~2ms without, so roughly 4ms of lock hold, and only on the call that first links two identities — the enricher takes the same lock on every merge at far higher volume, so this is a rounding error on that contention rather than a new bottleneck. Recording on each matched person instead would avoid the lock, at the cost of a second resolution mode in ``IdentityResolver``; not worth it until the lock actually shows up in production.