refactor: rename ErrActorPanic to ActorPanicError and add Unwrap

[joshua-temple]

What this change does

Renames the actor-panic error type ErrActorPanic to ActorPanicError and gives it an Unwrap() method, bringing it in line with its sibling panic error types — GuardPanicError, AssignPanicError, and ActionPanicError — which are all typed structs with Error() and Unwrap(). The ErrXxx prefix is conventionally reserved for sentinel error values; this type is a struct, so the old name was misleading. Unwrap() lets errors.As/errors.Is reach an underlying error when the recovered panic value is itself an error. The type now lives alongside its siblings in errors.go, and its field follows the sibling Recovered any shape.

Why

A consistent error surface before the 1.0 API freeze — renaming an exported type after 1.0 would be a breaking change.

Relates to: 1.0 stability hardening

Checklist

• Commits are signed off (git commit -s) per the DCO
• Conventional commit messages (type: subject)
• mage check passes locally — the state module passes lint, race tests, and govulncheck; the only mage check failure is the same pre-existing telemetry/datadog stdlib vulnerability noted above, unrelated to this change.
• Tests added/updated for the change
• Public API changes documented (godoc) and noted for the changelog