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/media-use/audio/scripts/lib/bgm.mjs

@@ -20,6 +20,13 @@ import { pythonInvocation } from "./python.mjs";
 const r3 = (x) => Number(x.toFixed(3));
 const lyriaKey = () => process.env.GEMINI_API_KEY || process.env.GOOGLE_API_KEY || "";
 
+// Default BGM level. Under narration music is a bed that must stay under the
+// voice — 0.12 linear ≈ -18 dB. A silent film (no voice) has no voice to duck
+// beneath, so BGM sits forward at 0.9. Callers may override per composition.
+export const BGM_BED_VOLUME = 0.12;
+export const BGM_SILENT_VOLUME = 0.9;
+export const bgmDefaultVolume = (hasVoice) => (hasVoice ? BGM_BED_VOLUME : BGM_SILENT_VOLUME);
+
 const BGM_PY_DEPS = ["transformers", "torch", "soundfile", "numpy"];
 const BGM_PY_PROBE =
   "import transformers, soundfile, torch, numpy; from transformers import MusicgenForConditionalGeneration";
@@ -51,7 +58,7 @@ export async function retrieveBgm({ query, headers, hyperframesDir, hasVoice })
   await downloadTo(top.audio_url, join(hyperframesDir, rel));
   return {
     path: rel,
-    volume: hasVoice ? 0.8 : 0.9,
+    volume: bgmDefaultVolume(hasVoice),
     query: q,
     mode: "retrieve",
     duration_s: typeof top.duration === "number" ? r3(top.duration) : null,
@@ -116,7 +123,7 @@ export function generateBgmDetached({
   mkdirSync(join(hyperframesDir, "assets", "bgm"), { recursive: true });
   const log = join(hyperframesDir, "assets", "bgm", `bgm-${Date.now()}.log`);
   const targetS = Math.max(1, durationS);
-  const baseMeta = { path: rel, mode: null, volume: hasVoice ? 0.8 : 0.9, pending: true };
+  const baseMeta = { path: rel, mode: null, volume: bgmDefaultVolume(hasVoice), pending: true };
 
   const lyriaConfigured = !!lyriaKey() && !!lyriaRecipe && existsSync(lyriaRecipe);