plugin: granular streaming hooks (P3, retargeted) — on-turn-start/end + on-message-update - #43

…te) (dirge-code#43)

Builds on P0's turn-boundary detection. Wires the new
AgentEvent::TurnStart / TurnEnd events into three new plugin hooks:

  on-turn-start     {:index N}
  on-message-update {:index N :partial "text-so-far"}
  on-turn-end       {:index N :message "full turn text"}

These give plugins per-turn observability that wasn't possible from
the previous `on-response` hook alone (which only fires once at the
end of the whole agent run). Typical uses:

* Per-turn cost / latency tracking.
* Token-level filtering or annotation.
* Cancel-on-condition based on streaming output.
* Streaming a live transcript to an external sink.

Implementation:

* `src/ui/streaming.rs` — new TokenBatcher type that collects tokens
  since the last flush and yields the accumulated text once the count
  threshold (DEFAULT_BATCH_TOKENS = 16) is crossed. Count-based
  rather than time-based so the tests are deterministic without
  mocking Instant. Module is cfg-gated to the plugin feature.
* `src/ui/mod.rs`:
  - New per-turn streaming state (token_batcher, current_turn_text,
    current_turn_index), cfg-gated.
  - AgentEvent::Token arm now pushes to the batcher; on flush,
    dispatches `on-message-update` with the accumulated text.
  - AgentEvent::TurnStart arm resets per-turn state and dispatches
    `on-turn-start`.
  - AgentEvent::TurnEnd arm flushes any trailing partial batch (as
    a final `on-message-update`), then dispatches `on-turn-end`
    with the full turn text.
* `src/main.rs` — adds the three hook names to auto-discovery.

`harness/replace-message` (mutating the persisted assistant text from
on-message-end) is deferred. It needs session-write position tracking
and interacts with usage accounting; will land in a follow-up phase.

Tests: 6 new unit tests on `TokenBatcher` — threshold yielding,
post-flush fresh batch, partial-batch draining, reset, zero-threshold
clamping, lossless content over a run. Total: 516 pass with plugin
(was 510); 460/12 baseline without plugin unchanged.

plugins/turn_timing.janet — example using on-turn-start / on-turn-end
to notify the user how long each turn took.

Refs dirge-87x.

Co-authored-by: Yogthos <yogthos@gmail.com>