chore(notebook): revert Expert Colab segmenter default to silero-v3.1

Keeps the scene detector at semantic (unchanged from 10f5b13), but
moves the speech segmenter default back from "ten" to "silero-v3.1"
to match the rest of the codebase:

- GUI ensemble tab (index.html L451 / L605) has silero-v3.1 selected
- CLI argparse defaults across standard and ensemble flags remain
  unset (None) which resolves to silero-v3.1 via resolver_v3

User requested this alignment just before the v1.8.11 release so the
notebook matches what everyone else sees.

Also updates the Colab Form dropdown choices from ["silero"] (which
resolves to v4.0 per silero backend default) to ["silero-v3.1"] so
the picker value is explicit and unambiguous.

Note: replacing generic "silero" with "silero-v3.1" in the dropdown is
intentional. The bare "silero" factory entry maps to version="v4.0"
in SileroSpeechSegmenter.__init__ (backends/silero.py:102), so a
notebook user picking "silero" would have gotten v4.0, not v3.1.
Explicit version pinning avoids that surprise.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: meizhong986

notebook/WhisperJAV_colab_edition_expert.ipynb

@@ -29,7 +29,7 @@
     "\n",
     "#@markdown **Expert Audio Setup**\n",
     "pass1_scene_detector = \"semantic\" #@param [\"automatic\", \"auditok\", \"silero\", \"semantic\"]\n",
-    "pass1_speech_segmenter = \"ten\" #@param [\"automatic\", \"silero\", \"ten\", \"none\"]\n",
+    "pass1_speech_segmenter = \"silero-v3.1\" #@param [\"automatic\", \"silero-v3.1\", \"ten\", \"none\"]\n",
     "pass1_speech_enhancer = \"none\" #@param [\"none\", \"ffmpeg-dsp\", \"clearvoice\", \"zipenhancer\", \"bs-roformer\"]\n",
     "#@markdown <font size=\"1\">auditok=energy (fast), silero=VAD, semantic=texture (complex audio) | enhancer: ffmpeg-dsp(no GPU), clearvoice(48k), bs-roformer(vocal)</font>\n",
     "\n",
@@ -52,7 +52,7 @@
     "\n",
     "#@markdown **Expert Audio Setup**\n",
     "pass2_scene_detector = \"semantic\" #@param [\"automatic\", \"auditok\", \"silero\", \"semantic\"]\n",
-    "pass2_speech_segmenter = \"ten\" #@param [\"automatic\", \"silero\", \"ten\", \"none\"]\n",
+    "pass2_speech_segmenter = \"silero-v3.1\" #@param [\"automatic\", \"silero-v3.1\", \"ten\", \"none\"]\n",
     "pass2_speech_enhancer = \"none\" #@param [\"none\", \"ffmpeg-dsp\", \"clearvoice\", \"zipenhancer\", \"bs-roformer\"]\n",
     "\n",
     "#@markdown **FFmpeg Filters** *(only if enhancer is ffmpeg-dsp)*\n",