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/tts.mjs

@@ -18,6 +18,7 @@ import { existsSync, mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync
 import { tmpdir } from "node:os";
 import { dirname, join } from "node:path";
 import { heygenAuthHeaders, heygenCredential, heygenJSON } from "./heygen.mjs";
+import { pythonInvocation } from "./python.mjs";
 
 // ── provider detection ────────────────────────────────────────────────────────
 export function heygenAvailable() {
@@ -25,7 +26,8 @@ export function heygenAvailable() {
 }
 export function elevenlabsAvailable() {
   if (!process.env.ELEVENLABS_API_KEY) return false;
-  const r = spawnSync("python3", ["-c", "import elevenlabs"], {
+  const { cmd, args } = pythonInvocation(["-c", "import elevenlabs"]);
+  const r = spawnSync(cmd, args, {
     stdio: "ignore",
   });
   return r.status === 0;
@@ -214,11 +216,14 @@ export async function synthesizeOne({
 }) {
   if (provider === "heygen") return synthesizeHeygen({ text, voiceId, lang, speed, wavAbs });
   if (provider === "elevenlabs") {
-    const r = await spawnP(
-      "python3",
-      ["-c", ELEVENLABS_PY, writeTmpText(text), voiceId, wavAbs],
-      {},
-    );
+    const { cmd, args } = pythonInvocation([
+      "-c",
+      ELEVENLABS_PY,
+      writeTmpText(text),
+      voiceId,
+      wavAbs,
+    ]);
+    const r = await spawnP(cmd, args, {});
     return { ok: r.status === 0 && existsSync(wavAbs), words: null };
   }
   // kokoro — via the published CLI; --output is relative to the project dir.