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/build-frame.mjs

@@ -37,6 +37,7 @@ import {
   parseFonts,
   pickAccent,
   semanticColors,
+  STATUS_ROLE_KEY,
   UA_DEFAULT_COLORS,
 } from "./lib/tokens.mjs";
 
@@ -253,11 +254,7 @@ if (brandColors.length && presetColors.length) {
     let next;
     if (val === prDark) next = mapDark;
     else if (val === prLight) next = mapLight;
-    else if (
-      /(?:^|[-_])(?:positive|negative|success|error|warning|danger|good|bad|up|down)(?:[-_]|$)/i.test(
-        key,
-      )
-    )
+    else if (STATUS_ROLE_KEY.test(key))
       // semantic status colors (green/red …) — the HUE carries the meaning; never repaint.
       // MUST precede the accent checks: a preset's red "negative" is often its 2nd-most-chromatic
       // color and would otherwise be claimed as accent2 and recolored to the brand hue.