Add bounded CPU VAD diagnostic with real model contract tests

tools/speech-vad/README.md

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+# CPU speech-activity diagnostic
+
+Research tooling, separate from searchi's normal GPU-capable runtime. No model
+download, transcription, source indexing or product default is changed.
+Use the frozen source and model hashes in [the protocol](../../docs/SPEECH-VAD.md).
+
+```sh
+cmake -S tools/speech-vad -B .searchi/speech/build-vad-diagnostic -DWHISPER_SOURCE=/absolute/path/to/pinned/whisper.cpp
+cmake --build .searchi/speech/build-vad-diagnostic --config Release --parallel 1
+```
+
+Run `searchi-vad-diagnostic MODEL PCM_F32LE`, retaining stdout and stderr
+separately. Independently verify the model SHA-256 before running. Input must
+be verified mono 16-kHz little-endian float32 PCM, at most ten minutes, with
+finite normalized samples. CPU and one inference thread are fixed. Native error
+logs invalidate detection even when upstream returns success. Probability
+count must match the pinned 512-sample inference framing.
+
+Output JSON retains every probability and raw centisecond interval, source
+sample count and load/detection/segmentation milliseconds. Interval padding is
+already applied. Preserve this raw output; a subsequent harness must explicitly
+intersect intervals with actual decoded source samples and add the verified
+source origin exactly once. Do not concatenate disjoint intervals without a
+piecewise source map. Zero intervals are a valid no-speech decision, distinct
+from nonzero exit status or missing JSON.
+
+The wrapper fixes the preregistered segmentation parameters: threshold 0.5,
+minimum speech 250 ms, silence 100 ms, maximum speech FLT_MAX, padding 30 ms,
+overlap 0.1 seconds. It does not establish model accuracy or a production
+speech gate. Compare the complete frozen speech/music/sound pool before judging
+quality; preserve original Google audio retrieval regardless of transcription.