Coalesce
/coalesce Tier 2
Cross-action request coalescing via a shared *Coalescer. Identical to Dedup but: (1) one coalescer can span multiple actions, (2) a canceled waiter exits cleanly while the in-flight call KEEPS RUNNING for other waiters.
Signature
action contract.Coalesce(c *Coalescer, keyFn func(Req) string) *Builder[Req, Res]