Add golden-audio STT tests against real whisper.cpp (#288)

Four committed WAV fixtures go through the real whisper.cpp binding in
cmd/mavsttd, so a wrong model, a wrong language hint, a broken resample
or a regressed silence gate fails `make test` instead of surfacing as
Maven mishearing him.

The fixtures are piper-synthesised, not recorded: scripts/gen-stt-fixtures.sh
drives the vendored piper with the ru_RU-irina voice Maven already speaks
with, so nothing of the owner's voice is committed and every fixture is
reproducible. 360K total for three Russian clips and one English.

Matching is tolerant on purpose. Golden transcripts move with the model,
so each case asserts intent-carrying keywords (prefix match, so Russian
inflection does not fail it) plus a word error rate ceiling, not an exact
string. The matcher is unit-tested on its own and needs no model.

TestGoldenAudioTranscription skips when models/stt/ggml-small.bin is
absent, so `make test` still passes on a box without models.
TestGoldenFixturesAreCanonical runs everywhere and checks the WAVs are
16k mono s16le and would clear mavsttd's own silence gate.

Makefile

@@ -16,7 +16,7 @@ PIPER_BIN     := $(shell pwd)/deps/piper/piper
 PIPER_MODEL   := $(shell pwd)/models/tts/ru_RU-irina-medium.onnx
 PIPER_ESPEAK  := $(shell pwd)/deps/piper/espeak-ng-data
 
-.PHONY: all build build-stt build-tts build-daemon build-client build-waked build-web build-poll build-caldav clean test fmt-check vet run-stt run-tts run-web download-embedder deps-go eval-router eval-recall eval-phrasing eval-models
+.PHONY: stt-fixtures test-stt-golden all build build-stt build-tts build-daemon build-client build-waked build-web build-poll build-caldav clean test fmt-check vet run-stt run-tts run-web download-embedder deps-go eval-router eval-recall eval-phrasing eval-models
 
 all: build
 
@@ -139,6 +139,17 @@ eval-models:
 	MAVEN_LLM_URL="$(MAVEN_LLM_URL)" $(GO) test -v -count=1 -timeout 60m \
 		-run TestLLMRouterBaseline ./internal/router/eval/
 
+# stt-fixtures — regenerate the golden STT audio in cmd/mavsttd/testdata from
+# the piper voices (#288). The committed WAVs are synthesised, never recorded,
+# so this is the only way they should ever change. TestGoldenAudioTranscription
+# then scores them against ggml-small; it self-skips when the model is absent.
+stt-fixtures:
+	./scripts/gen-stt-fixtures.sh
+
+test-stt-golden:
+	CGO_CFLAGS="$(CGO_CFLAGS)" CGO_LDFLAGS="$(CGO_LDFLAGS)" LD_LIBRARY_PATH="$(shell pwd)/deps/lib" \
+		$(GO) test -v -count=1 -run TestGolden ./cmd/mavsttd/
+
 run-stt: build-stt
 	LD_LIBRARY_PATH="$(shell pwd)/deps/lib" \
 		./mavsttd -socket /tmp/maven/stt.sock -model $(WHISPER_MODEL)