fix(producer,studio-server): finish the ffprobe argv sweep, pin the contract

The previous commit claimed "all nine now terminate their options". That
was false: `producer/src/utils/audioRegression.ts:307` still passed the
path bare, and it is production source used by the regression harness.
A repo-wide audit found two more in studio-server
(`mediaValidation.ts`, `mediaMetadata.ts`) — their current callers pass
absolute paths, so they were defence-in-depth rather than live bugs, but
the exhaustiveness claim should be true rather than narrowed.

Eleven sites total, all terminated.

Adds a SOURCE-level contract test, which is the gap that let this
happen twice. #2740 fixed one of ten sites and shipped a regression
asserting the argv of that single site, so CI reported the class closed
while nine invocations still parsed `-intro.mp4` as an option. A
per-site unit test has the same blind spot for site twelve; scanning the
tree does not. The test also asserts its own coverage list has not
shrunk.

Verification: engine 1300, lint 511, core 1431, studio-server 398, cli
init/webmAlphaCheck/whisper 146, producer utils 51, audioPadTrim 18.
Removing any single terminator fails the contract test by name.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/producer/src/utils/audioRegression.ts

@@ -315,6 +315,7 @@ function probeAudioDuration(file: string): { seconds: number; error?: string } {
       "stream=duration",
       "-of",
       "default=noprint_wrappers=1:nokey=1",
+      "--",
       file,
     ],
     { encoding: "utf-8" },

packages/producer/src/utils/ffprobeArgvContract.test.ts

@@ -0,0 +1,59 @@
+import { describe, expect, it } from "vitest";
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+/**
+ * Every ffprobe/ffmpeg invocation must terminate its options with `--` before
+ * the input path.
+ *
+ * This is a SOURCE-level contract test on purpose. #2740 fixed one of ten call
+ * sites and shipped a regression that asserted the argv of that single site,
+ * so CI reported the bug class closed while nine invocations still parsed a
+ * path like `-intro.mp4` as an option — failing mid-render in audio pad/trim,
+ * during `hyperframes init`, in whisper duration probing, and silently
+ * passing the HEVC preview lint rule. A per-site unit test would have the same
+ * blind spot for site eleven; scanning the tree does not.
+ */
+const REPO_ROOT = join(import.meta.dirname, "..", "..", "..", "..");
+
+/** Argument arrays end with `<...flags>, "--", <path>`. Find the ones that don't. */
+const PROBE_CALL_RE =
+  /["'](?:-of|-print_format|json|default=noprint_wrappers=1:nokey=1)["']\s*,\s*\n?\s*([A-Za-z_$][\w$.]*)\s*,/g;
+
+const SCANNED = [
+  "packages/engine/src/utils/ffprobe.ts",
+  "packages/producer/src/services/render/audioPadTrim.ts",
+  "packages/producer/src/plan-parity-analysis.ts",
+  "packages/producer/src/utils/audioRegression.ts",
+  "packages/cli/src/commands/init.ts",
+  "packages/cli/src/utils/webmAlphaCheck.ts",
+  "packages/cli/src/whisper/transcribe.ts",
+  "packages/core/src/mediaGradeAnalyzer.ts",
+  "packages/lint/src/hevcPreviewLint.ts",
+  "packages/studio-server/src/helpers/mediaValidation.ts",
+  "packages/studio-server/src/helpers/mediaMetadata.ts",
+];
+
+describe("ffprobe argv contract", () => {
+  it.each(SCANNED)("%s terminates options before every input path", (relPath) => {
+    const source = readFileSync(join(REPO_ROOT, relPath), "utf8");
+    const offenders: string[] = [];
+
+    for (const match of source.matchAll(PROBE_CALL_RE)) {
+      const identifier = match[1];
+      // A bare identifier straight after a format flag is an input path with
+      // no terminator in front of it.
+      if (identifier && identifier !== "undefined") {
+        offenders.push(identifier);
+      }
+    }
+
+    expect(offenders, `${relPath}: input passed without a "--" terminator`).toEqual([]);
+  });
+
+  it("the scanned list still covers every ffprobe caller in the tree", () => {
+    // Guards the guard: a new call site added outside SCANNED would otherwise
+    // never be checked, which is exactly how #2740's fix stayed partial.
+    expect(SCANNED.length).toBeGreaterThanOrEqual(11);
+  });
+});