stable-timestamps: per-segment VAD decoding for subtitle-quality timestamps

Replace concatenate-decode-remap pipeline with per-segment VAD decoding,
matching how stable-ts/faster-whisper works. Each VAD speech segment is
decoded independently and timestamps are offset by the segment's original
start time — no mapping table or interpolation needed.

Results on 5-min synthetic audio (46 utterances, 7x 20s pauses):
  pct_words_overlap: 0.89% (vs 5.7% stable-ts, 22.6% previous v2)
  n_words_overlap:   5     (vs 22  stable-ts, 144   previous v2)
  Wall time:         22.8s (vs 43.2s stable-ts — 1.9x faster via Metal)

Code removed:
- whisper_vad() concatenation + mapping table building
- vad_time_mapping struct, vad_mapping_table, has_vad_segments from state
- map_processed_to_original_time() in whisper.cpp
- whisper_stable_map_processed_to_original() in whisper-stable.cpp
- mapping params from whisper_stable_snap_segments()

Code added:
- whisper_full_vad_segments(): ~70-line per-segment decode loop
- whisper_full_parallel() with VAD delegates to whisper_full()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thewh1teagle

examples/cli/cli.cpp

@@ -295,7 +295,7 @@ static void whisper_print_usage(int /*argc*/, char ** argv, const whisper_params
     // Voice Activity Detection (VAD) parameters
     fprintf(stderr, "\nVoice Activity Detection (VAD) options:\n");
     fprintf(stderr, "             --vad                           [%-7s] enable Voice Activity Detection (VAD)\n",            params.vad ? "true" : "false");
-    fprintf(stderr, "             --stable-timestamps             [%-7s] enable stable timestamps (requires --vad-model)\n", params.stable_timestamps ? "true" : "false");
+    fprintf(stderr, "             --stable-timestamps             [%-7s] enable stable timestamps\n", params.stable_timestamps ? "true" : "false");
     fprintf(stderr, "  -vm FNAME, --vad-model FNAME               [%-7s] VAD model path\n",                                   params.vad_model.c_str());
     fprintf(stderr, "  -vt N,     --vad-threshold N               [%-7.2f] VAD threshold for speech recognition\n",           params.vad_threshold);
     fprintf(stderr, "  -vspd N,   --vad-min-speech-duration-ms  N [%-7d] VAD min speech duration (0.0-1.0)\n",                params.vad_min_speech_duration_ms);
@@ -1005,12 +1005,6 @@ int main(int argc, char ** argv) {
         exit(0);
     }
 
-    if (params.stable_timestamps && params.vad_model.empty()) {
-        fprintf(stderr, "error: --stable-timestamps requires --vad-model\n");
-        whisper_print_usage(argc, argv, params);
-        return 2;
-    }
-
     if (params.no_prints) {
         whisper_log_set(cb_log_disable, NULL);
     }