Replace the onEnter event-name convention with first-class state entry hooks

[camcima]

Finding #6 of the 2026-08 architecture review, carried over from the June 2026 review's design note. Deferred from #52.

Problem

OnEnterObserver implements "run something when entering state X" as a magic event name layered on ordinary transition events. To give approved an entry hook you must declare a sentinel self-transition (docs/observers.md):

.addTransition("approved", "approved", { event: "onEnter" })

That has real costs:

• Graph pollution. Sentinel self-loops appear in GraphBuilder output as genuine edges, so every DOT/Mermaid export of a workflow with entry hooks is wrong as documentation — it shows transitions the domain does not have.
• Name collision. A workflow with a real domain event called onEnter silently becomes an entry hook (or vice versa). The convention has no namespace.
• It leans on a subtle semantic. The mechanism works because event-driven self-transitions fire event observers without moving state — intentional and tested, but non-obvious, and it means entry hooks ride on an edge case rather than on a first-class concept.
• Ordering is implicit. The hook is enqueued as a separate top-level operation, so "on entry" really means "some time after the operation that entered it drains". The ifStateName guard added in v4 fixed the wrong-state bug, but the timing is still surprising for anyone reading onEnter as a lifecycle hook.

Proposed direction

Declare entry hooks at build time as their own concept, keyed on the state rather than on an event name:

new ProcessBuilder("order")
  .addState("approved", { onEnter: [chargeCard] })
  // or: .addOnEnter("approved", chargeCard)

A generic after-observer then dispatches on frame.toState, with no sentinel edge in the graph and no event-name namespace to collide with. Decide explicitly, and document, whether hooks run inline within the committing operation or as a chained operation as they do today — the current asynchronous behavior is defensible, but it should be a stated contract rather than a side effect of the implementation.

OnEnterObserver can remain as a deprecated shim for one major if that eases migration.

Acceptance criteria

• Entry hooks are declarable on a state without a sentinel transition.
• Graph exports contain no artificial self-loops for states with entry hooks.
• An event genuinely named onEnter no longer behaves differently from any other event.
• Execution timing (inline vs chained) is documented as a contract and pinned by tests.
• Migration guide entry under docs/migration/.