fix(skills): pipeline fixes from prompt-guide validation (BGM, caption accent, voice, PR version)

Behavior fixes surfaced by the prompt-guide validation campaign (Tier 1+2
of the upstream bug list; Tier 3 tracked in #2107). Split out from the
doc-only updates, which follow in a separate PR.

- BGM level: default bed volume under narration was 0.8 linear (~-2 dB,
  ~16 dB too hot vs voice). Now 0.12 (~-18 dB) via shared
  bgmDefaultVolume() in media-use bgm.mjs + assemble-index fallbacks in
  faceless-explainer / pr-to-video / product-launch-video. Explicit
  volume still wins; silent-film 0.9 and music-to-video unchanged.
  Adds bgm.test.mjs (3 cases); bgm.md reference updated to match.
- Caption accent: semanticColors() ranked accents purely by chroma, so a
  preserved status red (#dc2626) outranked the brand accent and captions
  highlighted in error-red. Status-keyed colors now excluded via shared
  STATUS_ROLE_KEY regex consumed by both tokens.mjs and build-frame.mjs
  (all three skill copies kept in sync).
- Voice threading: workflow SKILL.md Step 3.1 blocks now instruct
  choosing the narration voice from the user's ask and passing
  --voice <id>; previously "a male voice" was silently ignored and the
  default (Marcia/am_michael) always won.
- fetch-pr shipping version: MERGED PRs get best-effort shipped_version
  + version_source in pr.json (first release published at/after merge,
  else default-branch package.json marked unreleased); ingest surfaces
  it as a 'Shipped in:' brief line; story-design.md forbids inventing
  versions when absent.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skills/product-launch-video/scripts/assemble-index.mjs

@@ -23,7 +23,7 @@
 // video, frames only). Durations come from STORYBOARD (audio sync-durations
 // writes them), NOT from here; this file carries only media PATHS, keyed by
 // frame number:
-//   { "bgm":   { "path": "assets/bgm/x.mp3", "volume": 0.8 } | null,
+//   { "bgm":   { "path": "assets/bgm/x.mp3", "volume": 0.12 } | null,
 //     "voices":[ { "frame": 3, "path": "assets/voice/03.wav" } ],
 //     "sfx":   [ { "frame": 3, "file": "assets/sfx/x.mp3", "offset_s": 0,
 //                  "duration_s": 1.0, "volume": 0.35 } ] }
@@ -388,7 +388,9 @@ if (audio.bgm?.path) {
         `bgm is ${cov.dur?.toFixed?.(1) ?? "?"}s (< ${TOTAL}s) and could not be extended (${cov.reason}) — the tail will be silent; install ffmpeg`,
       );
     }
-    const vol = audio.bgm.volume != null ? audio.bgm.volume : voiceCount > 0 ? 0.8 : 0.9;
+    // An explicit volume from audio_meta always wins. Otherwise BGM under
+    // narration is a bed (0.12 ≈ -18 dB); a silent film sits it forward at 0.9.
+    const vol = audio.bgm.volume != null ? audio.bgm.volume : voiceCount > 0 ? 0.12 : 0.9;
     body.push(
       `      <!-- BGM -->`,
       `      <audio`,