Hear speech instead of loudness in mavwaked (V-487)

silero-vad replaces the energy threshold when -vad-model points at it.
Everything after the speech decision is the same state machine: the speech
hold, the silence hold, the length cap and the utterance buffer.

The model window is 512 samples and the capture frame is 480, so silero.go
re-chunks across frames. main.go claimed the two matched, which was true of
silero v4.

Stage two, the wake word, is not here. It needs a Russian keyword model that
does not exist yet.

docs/evals/2026-08-09-silero-vad.md

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+# silero-vad against the energy threshold in mavwaked
+
+*Measured 2026-08-09 on homesrv. V-487, stage one of two.*
+
+mavwaked decided an utterance had started by comparing frame energy to an
+adaptive floor. That answers "is this frame loud". A fan, a door and a
+television are all loud, and every utterance mavwaked accepts becomes a turn.
+
+silero-vad answers "is this frame speech". It is 2.3MB of ONNX and it replaces
+the comparison and nothing else. The speech hold, the silence hold, the length
+cap and the utterance buffer are the same state machine either way.
+
+## What it declines
+
+Speech is the four piper fixtures `mavsttd` already scores against, so nothing
+of the owner's voice is committed. Non-speech is white noise at the same RMS as the clip beside it. That is the
+cheapest thing that fools an energy floor.
+
+| clip | silero, speech frames on speech | silero on noise | energy on noise |
+|---|---|---|---|
+| ru_fact.wav | 59 | 0 | 68 |
+| ru_query.wav | 69 | 0 | 79 |
+| ru_reminder.wav | 80 | 0 | 89 |
+| en_act.wav | 90 | 0 | 99 |
+
+The energy threshold accepts every noise clip as a complete utterance. Silero
+calls not one frame of any of them speech, and still hears all four spoken
+clips. `TestSileroHearsSpeechAndDeclinesNoise` is that table.
+
+White noise is a floor, not a proof. It says nothing about a television, which
+is speech, or about a fan, which is narrowband. Those need room recordings and
+this box has none.
+
+## What it costs
+
+`BenchmarkSileroFrame` on the homesrv laptop (Ryzen 5 5600U), one 30ms frame
+through the model including the re-chunking:
+
+    509µs per frame
+
+That is 1.7% of one core, on the slower of the two machines. The detector runs
+on the workstation beside the microphone, never on the GPU. This number is what
+says it does not need one.
+
+## The window is 512 samples, not 480
+
+`cmd/mavwaked/main.go` claimed the frame contract matched silero's input
+exactly. That was true of silero v4. Version 5 takes exactly 512 samples at 16kHz, plus 64 samples of context from
+the previous window. So `sileroVAD` buffers across capture frames, and a frame
+completing no window inherits the previous probability. `TestSileroRechunksAcrossFrames` pins it.
+
+## Still an energy gate by default
+
+`-vad-model` is empty in the code default, so a deployment that does not pass
+it runs exactly what shipped before. Barge-in is untouched and deliberately so. It reads frame energy while she is
+speaking, which is a different question from whether the frame is speech.
+
+## Not done here
+
+The wake word. This is stage one of the two V-487 asks for. The second needs a
+keyword model that does not exist yet. The pretrained openWakeWord keywords are
+English, and a Russian one has to be trained. Until then anything spoken near
+the microphone still becomes a turn. It is now merely required to be speech.