refactor: pre-1.0 state API consistency (Tick shape + idiomatic error names)

[joshua-temple]

What this change does

Two pre-1.0 breaking consistency fixes on the state public API, landed before the contracts freeze:

• ServiceRunner.Tick returns []FireResult like the other drivers. It was the odd one out ((FireResult, bool)); Scheduler.Tick and ActorSystem.Tick already return a slice. Callers now check len(results) > 0. A return type can't be unified additively later, so this had to happen pre-freeze.
• Error types now use the idiomatic *Error suffix. The 15 Err*-prefix structs plus MultiRegionErr are renamed to the *Error suffix (ErrInvalidTransition to InvalidTransitionError, and so on), matching VerifyError/SnapshotError/WaitTimeoutError and Go convention. The Err* prefix is the convention for sentinel error values; these are struct types you errors.As into, and there are no sentinel values in the package.

Why

These are the two now-or-never items from the pre-1.0 API audit: both are breaking, so they belong before the freeze rather than as a future v2 break. Idiomatic error naming also signals a hand-built, conventional library.

Checklist

• Commits are signed off (git commit -s) per the DCO
• Conventional commit messages (type: subject)
• Every workspace consumer (durable, cluster, e2e, transport, wasm, sink, examples, source*) builds and go vets clean; state tests green
• Public API changes documented (godoc + CHANGELOG under the staged [1.0.0])