feat(capi): ABI v5 - surface <EOU> vs <EOB> distinction across the C boundary

[richiejp]

EOU and EOB are treated as the same thing, but they are very different.
The consumer may want to ignore messages marked as EOB altogether.
So this separates them in the C API.

---

The streaming C API conflated end-of-utterance and backchannel: the feed
eou_out flag and the JSON "eou" field were 1 for either token, so a voice
agent could not tell "the user yielded the turn" (, respond) from
"the user acknowledged while listening" (, do not treat as a turn).

• parakeet_capi_stream_feed's *eou_out is now an event bitmask
  (PARAKEET_EVENT_EOU | PARAKEET_EVENT_EOB) instead of an any-event 0/1
• the streaming JSON documents carry separate "eou"/"eob" 0/1 flags plus
  an "events" array ({"type":"eou"|"eob","frame","t"}) with per-event
  timestamps
• new parakeet_capi_stream_drain_events / parakeet_capi_free_events
  expose typed per-event records (token, is_eob, encoder_frame,
  time_sec) on the flat API; the queue is shared with the JSON entry
  points
• StreamingSession gains a non-draining events() peek; the C layer
  watermarks the queue to attribute events to one feed pass (a feed can
  span several encoder chunks, which last_chunk_had_eou() cannot express)
• decoder reset-on-/ is deliberately unchanged (matches NeMo's
  reference driver); the distinction is a downstream turn-taking signal
• tests: both C-API stream tests gain a two-utterance-clip phase (shared
  builder in tests/stream_clips.hpp, same construction as
  gen_stream_reset_baseline.py) asserting the typed mid-stream and
  exact flag/mask agreement with the drained queue

Verified against mudler/parakeet-cpp-gguf realtime_eou_120m-v1-f16: all
61 ctest targets pass; the two-utterance clip yields the mid-stream
at t=7.84s with mask=PARAKEET_EVENT_EOU.

Assisted-by: Claude:claude-fable-5 [Claude Code]