flowcat-core: opt-in full-duplex (barge-in) support for the cascaded pipeline

[rolandknight]

Closes #60.

Adds an opt-in full-duplex path for the cascaded pipeline with working barge-in, validated live end-to-end (str0m WebRTC + aiortc client, whisper.cpp STT, OpenRouter LLM with tool calls, Kokoro TTS). The stock half-duplex builder and its TurnMute behavior are untouched; nothing changes unless you call the new builder.

The problem (details in #60)

1. The runtime intercepts Frame::Interruption (drain + forward) and never delivers it to process_frame — the existing Interruption arms in the transport sinks are unreachable, so frame-level barge-in cannot work on the cascaded path.
2. Nothing on the cascaded path emits BotStartedSpeaking/BotStoppedSpeaking, so VadProcessor's barge-in gate never arms.
3. Even delivered, the frame path stalls behind any mid-await hop — we measured detection→sink delivery of 14 ms to 2.1 s depending on TTS/LLM activity — and an in-flight LLM stream cannot be cancelled, so the interrupted reply is spoken afterwards anyway.

The changes

Piece	What it does
FrameProcessor::on_interruption() (new, default no-op)	Runtime calls it in the Interruption arm after draining — the real delivery path for barge-in reactions
VadProcessor::{with_interrupt_flag, with_interrupt_notify}	Generation counter + Notify bumped synchronously at detection (before the broadcast)
LlmProcessor::with_interrupt_flag	Cooperative cancel between streamed chunks; closes response framing on cancel
AssistantContextAggregator::on_interruption	Keeps the partial reply in context, drops the open span (late LlmResponseEnd can no longer speak the reply)
SpeechGate (new)	VAD-edged segmentation: 300 ms pre-roll at the rising edge, all-zero flush marker (SPEECH_GATE_FLUSH_SAMPLES) at the falling edge — without the turn lock, fixed-window batch STT hallucinates turns on silence and splits utterances
BotSpeakingNotifier (new) + sink wiring	Emits bot-speaking edges into the pipeline head from playout tracking; arms the VAD gate
Out-of-band interrupt reactor + stale-audio latch	Notify-woken task flushes the carrier immediately (~110 µs from detection in our runs); the latch drops TTS audio that outran its interruption
build_cascaded_call_duplex (new, exported)	Assembles all of the above; generic over VadAnalyzer so it stays feature-agnostic (caller passes e.g. SileroVad)

Validation

• Live barge-in test (long spoken reply interrupted mid-playback over WebRTC): bot audio stops within the harness' 1 s budget consistently; interrupted reply does not resume; the follow-up turn (a tool call) executes normally. Repeated 3× + a 7-test regression suite, all green.
• cargo test -p flowcat-core --lib: 302 passed. cargo clippy -p flowcat-core --lib -- -D warnings: clean. cargo fmt applied.

Notes for review

• The reactor + latch exist because of measured need (the 2.1 s frame-path stall); if you'd rather solve preemption differently (e.g. cancellable process_frame), the hook + flag still stand alone.
• One default worth a look while you're here: VAD_MIN_VOLUME = 0.6 gated out moderate-volume speech entirely in our runs (only the loudest tail of utterances passed); we had to run with 0.2. Left untouched here since it's pipecat parity.
• Happy to split this into smaller PRs (hook/runtime fix first) if preferred.

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