sweep: name voice-daemon magic numbers, drop dead keep-alive vars (V-581)

mavsttd/whisper_handler.go: name the no_speech_prob confidence-zeroing
floor (0.9) and the whisper thread count (4), both previously bare
literals with no reason attached.

mavttsd/piper_handler.go: name piper's render rate (22050) and the
canonical wire rate (16000) used by the resampler, instead of repeating
the two numbers inline four times.

mavenclient/main.go: remove the strconv/io/net/time imports and their
`var _ = ...` keep-alive lines — dead weight with no caller, not future
scaffolding.

Behaviour-preserving; no test changed. go test -race ./internal/...
./cmd/... is green.

cmd/mavenclient/main.go

@@ -31,15 +31,11 @@ import (
 	"errors"
 	"flag"
 	"fmt"
-	"io"
 	"log"
-	"net"
 	"os"
 	"os/signal"
 	"path/filepath"
-	"strconv"
 	"syscall"
-	"time"
 
 	"github.com/kami/maven/internal/audio"
 	"github.com/kami/maven/internal/voice"
@@ -155,9 +151,3 @@ func writeWAV(path string, a audio.Audio) error {
 // jsonUnmarshal — kept local rather than pulling encoding/json into main.go
 // top-level space.
 func jsonUnmarshal(b []byte, v any) error { return json.Unmarshal(b, v) }
-
-// keep strconv + io + net + time alive for future duration/size helpers.
-var _ = strconv.Atoi
-var _ io.Reader = (io.Reader)(nil)
-var _ = net.IPv4
-var _ = time.Second