feat(engine): add on_enter node-entry hook

[wavekat-eason]

Adds FlowEffects::on_enter(node, kind), called once as the engine
enters each node in run_node, before that node's own effects run.
Default no-op, so it's backward-compatible — existing FlowEffects
impls compile and behave unchanged.

Why

A consumer (wavekat-voice's live call-screening UI) needs to light up
the current node on the flow graph as a call moves through it. The
existing effect calls (speak, collect_digit, record_message, …)
can't drive that on their own: node kinds repeat (multiple greetings /
menus), so an effect can't name which node is active, and an
effect-less node like hours fires no effect at all and would be
skipped on the map. on_enter carries the node's id and kind at
the one moment that unambiguously identifies it — arrival.

It fires for every visited node, including hours, and exactly once
per entry (a node re-entered by a menu retry loop fires again on each
real re-entry, not on internal retries).

Changes

• FlowEffects::on_enter(&mut self, node: &NodeId, kind: &'static str)
  with a default no-op.
• run_node calls fx.on_enter(id, kind) right after resolving the
  node's kind.
• MockEffects records entries; new test
  on_enter_reports_each_node_as_the_engine_reaches_it pins entry
  order across the open/closed paths and the menu-retry case.
• Version bump 0.0.4 → 0.0.5.

Release

The downstream consumer pins wavekat-flow = "0.0.5" behind a local
[patch.crates-io] for iteration. Publish 0.0.5 to crates.io after
this merges, so the consumer can drop the patch and resolve the pin
from the registry.

36 tests pass.