refactor: pre-1.0 state API naming and signature polish

[joshua-temple]

What this change does

Pre-1.0 naming and signature polish on the state public API, from an early-adopter audit. Four focused changes (atomic commits), all pre-tag so they stay out of the frozen 1.0 surface:

• Invoke outcomes are now options. Invoke(src, ...InvokeOption) / InvokeActor(src, ...InvokeOption) with WithInvokeOnDone(E) / WithInvokeOnError(E), mirroring the existing WithSpawnOnDone/OnError. This was the one place the project broke its own functional-options pattern; moving outcomes off the positional signature lets onCancel / fire-and-forget arrive additively instead of as a future break. Behavior is unchanged for callers that set both.
• Reducer registration vs. assign wiring are now distinct words. Registry.Assign is renamed to Registry.Reducer (matching Builder.Reducer), and Assign is reserved for the transition-wiring verb. This mirrors the existing Guard/When and Action/Do register-noun / wire-verb split, so no single word means two things. The AssignFn type is unchanged.
• StopChild collapsed into StopActor. Both emitted the identical StopActor effect; one verb now, aligned with Spawn/SpawnActor.
• Driver advance verbs converge on Tick. ServiceRunner.Run and ActorSystem.Step become Tick, matching Scheduler.Tick and the already-consistent Absorb/Pending/SettleDone/SettleError set.

Plus godoc cross-references across the Fire / FireSeq / FireEach family.

Why

The audit found the surface in good shape; these are the few items genuinely worth fixing before contracts freeze, biased toward extensibility and developer experience. The Verify rename (PR #139) set the precedent: favor discoverability and additive-friendly signatures over cleverness.

Note: the Tick verb is unified across the three drivers, but their return shapes intentionally still differ (ServiceRunner.Tick settles one service and returns (FireResult, bool); the others return []FireResult). Unifying return shapes is a separate, larger call left for later.

Checklist

• Commits are signed off (git commit -s) per the DCO
• Conventional commit messages (type: subject)
• state build/test/vet green; source + source/statemachine (the only consumer) green; docsgen clean
• Public API changes documented (godoc, docs site, CHANGELOG under the staged [1.0.0])