fix(hyperframes-media): resolve python3 -> python/py on Windows (#1922)

The audio engine shells out to `python3` for ElevenLabs TTS
(tts.mjs) and the local Lyria/MusicGen BGM paths (bgm.mjs). `python3`
is correct on macOS/Linux, but a standard python.org install on
Windows only creates `python.exe` plus the `py` launcher -- there is
no `python3.exe` (only the Microsoft Store build adds one). So every
`spawn("python3", ...)`/`spawnSync("python3", ...)` ENOENTs on a normal
Windows Python setup, silently disabling all Python-backed audio
features until the user hand-creates a `python3.exe` shim (reported: a
user copied python.exe to python3.exe to work around it, and separately
another had to target a python3 stub specifically).

Fix: a shared lib/python.mjs resolver probes the platform's candidates
in order and returns the argv prefix that actually launches Python 3 --
`["python3"]` / `["python"]` / `["py", "-3"]` on win32, `["python3"]`
then `["python"]` elsewhere -- resolved once per process. All direct
`python3` spawn sites in tts.mjs (elevenlabs probe + synth) and bgm.mjs
(pyOk probe, Lyria recipe, MusicGen script) now route through it. On
macOS/Linux `python3` still wins first, so behavior there is unchanged;
if nothing probes OK the resolver falls back to `python3` so the spawn
fails loudly exactly as before, never worse.

Scope: only the direct python3 invocations. bgm.mjs's pipInstall()
still shells `pip` -- switching that to `<python> -m pip` is the
separate concern of the open PR #1894 (draft); noting the overlap so
the two don't collide. Windows-specific whisper.cpp-vs-openai-whisper
detection and the npm_execpath/npx spawn issue from the same report are
distinct root causes, not addressed here.

Test: python.test.mjs (node:test) covers every platform/probe branch
with an injected probe -- no real interpreter spawned: non-win32 picks
python3; win32 prefers python3, falls back to python, then to `py -3`;
the py launcher is probed as `py -3 --version`; nothing-runs falls back
to the canonical python3; and pythonInvocation keeps the launcher's -3
ahead of caller args. Existing tts.spawn.test.mjs still passes (6/6).

skills/hyperframes-media/scripts/lib/bgm.mjs

@@ -15,6 +15,7 @@ import { spawn, spawnSync } from "node:child_process";
 import { existsSync, mkdirSync, openSync, closeSync } from "node:fs";
 import { join } from "node:path";
 import { downloadTo, searchSounds } from "./heygen.mjs";
+import { pythonInvocation } from "./python.mjs";
 
 const r3 = (x) => Number(x.toFixed(3));
 const lyriaKey = () => process.env.GEMINI_API_KEY || process.env.GOOGLE_API_KEY || "";
@@ -26,7 +27,8 @@ const LYRIA_PY_DEPS = ["google-genai", "python-dotenv"];
 const LYRIA_PY_PROBE = "import google.genai";
 
 function pyOk(probe) {
-  return spawnSync("python3", ["-c", probe], { stdio: "ignore" }).status === 0;
+  const { cmd, args } = pythonInvocation(["-c", probe]);
+  return spawnSync(cmd, args, { stdio: "ignore" }).status === 0;
 }
 function pipInstall(deps) {
   return spawnSync("pip", ["install", "-q", ...deps], { stdio: "ignore" }).status === 0;
@@ -120,11 +122,16 @@ export function generateBgmDetached({
 
   const fd = openSync(log, "w");
   if (useLyria) {
-    const proc = spawn(
-      "python3",
-      [lyriaRecipe, "--output", abs, "--duration", String(targetS), "--prompt", prompt],
-      { detached: true, stdio: ["ignore", fd, fd] },
-    );
+    const { cmd, args } = pythonInvocation([
+      lyriaRecipe,
+      "--output",
+      abs,
+      "--duration",
+      String(targetS),
+      "--prompt",
+      prompt,
+    ]);
+    const proc = spawn(cmd, args, { detached: true, stdio: ["ignore", fd, fd] });
     proc.unref();
     closeSync(fd);
     return {
@@ -141,7 +148,8 @@ export function generateBgmDetached({
     const seedS = Math.min(Math.max(seedSeconds, 10), 30);
     const loops = targetS > seedS ? Math.ceil(targetS / seedS) : 1;
     const script = musicgenScript({ prompt, abs, targetS, seedS });
-    const proc = spawn("python3", ["-c", script], { detached: true, stdio: ["ignore", fd, fd] });
+    const { cmd, args } = pythonInvocation(["-c", script]);
+    const proc = spawn(cmd, args, { detached: true, stdio: ["ignore", fd, fd] });
     proc.unref();
     closeSync(fd);
     return {