fix(hyperframes-media): surface a clear error when npx can't be resolved on Windows (#1961)

On Windows, resolveSpawnCommand routes `npx` through node + npm's
npx-cli.js (avoiding the un-spawnable npx.cmd), locating that CLI via
npm_execpath. When the script is run directly with `node audio.mjs`
instead of through npm/npx, npm_execpath is unset, so resolution returns
null and spawnP short-circuited to `{status:-1}` — silently. With
stdio:"ignore" hiding everything, callers just reported "TTS failed -
omitted" for every single line, giving no hint that the real cause was
an unresolvable npx. Debugging required reading the source.

Fix: when spawnP hits that null-resolution path, emit a clear one-time
diagnostic naming npm_execpath and the remedy (run via npx/npm, or export
npm_execpath) before returning {status:-1}. One-shot latch so a batch of
lines logs it once, not per line. Behavior is otherwise unchanged — still
returns {status:-1} and spawns nothing.

Test: new tts.spawn.test.mjs case — two consecutive win32 npx calls with
npm_execpath unset both return {status:-1}, nothing is spawned, and the
diagnostic (mentioning npm_execpath) is emitted exactly once. Existing
spawn tests unchanged (7/7 pass).

skills/hyperframes-media/scripts/lib/tts.spawn.test.mjs

@@ -1,7 +1,12 @@
 import { test } from "node:test";
 import assert from "node:assert/strict";
 import { EventEmitter } from "node:events";
-import { resolveNpxCliFromNpmExecPath, resolveSpawnCommand, spawnP } from "./tts.mjs";
+import {
+  resolveNpxCliFromNpmExecPath,
+  resolveSpawnCommand,
+  spawnP,
+  _resetNpxResolutionWarnForTests,
+} from "./tts.mjs";
 
 // Regression: on Windows, npx resolves to npx.cmd, which spawn() cannot exec
 // without shell:true — it fails ENOENT, silently swallowed as ok:false by the
@@ -95,3 +100,44 @@ test("spawnP does not enable shell for non-npx commands even on win32", async ()
   assert.deepEqual(captured[0].args, ["-c", "pass"]);
   assert.equal(captured[0].opts.shell, undefined);
 });
+
+// Regression: win32 + npx with npm_execpath unset can't locate the npx JS CLI,
+// so resolveSpawnCommand returns null and spawnP short-circuits. Previously it
+// returned {status:-1} silently — every TTS line just dropped as "TTS failed -
+// omitted" with no hint. Now it must surface a clear one-time diagnostic naming
+// npm_execpath, while still returning {status:-1} without spawning anything.
+test("spawnP surfaces a clear diagnostic (once) when npx can't be resolved on win32", async () => {
+  _resetNpxResolutionWarnForTests();
+  const errors = [];
+  const originalError = console.error;
+  console.error = (msg) => errors.push(msg);
+  const captured = [];
+  const emptyEnv = {}; // no npm_execpath
+  try {
+    const r1 = await spawnP(
+      "npx",
+      ["hyperframes", "tts"],
+      {},
+      "win32",
+      fakeSpawn(captured),
+      emptyEnv,
+      () => false,
+    );
+    const r2 = await spawnP(
+      "npx",
+      ["hyperframes", "tts"],
+      {},
+      "win32",
+      fakeSpawn(captured),
+      emptyEnv,
+      () => false,
+    );
+    assert.equal(r1.status, -1);
+    assert.equal(r2.status, -1);
+    assert.equal(captured.length, 0, "must not spawn anything when resolution fails");
+    assert.equal(errors.length, 1, "diagnostic is emitted once for a batch, not per line");
+    assert.match(errors[0], /npm_execpath/);
+  } finally {
+    console.error = originalError;
+  }
+});