fix: preserve explicit zero caption grain

skills/embedded-captions/scripts/make-theme.cjs

@@ -8537,7 +8537,7 @@ if (!bodyInBg || fx.html || fx.js || setp.fgHtml) {
 }
 
 // _postfx.sh: plate reaction after the matte composite (subject+text move as one)
-const P = HEROLESS ? { grain: (dna.plate || {}).grain || 5 } : dna.plate || {};
+const P = HEROLESS ? { grain: (dna.plate || {}).grain ?? 5 } : dna.plate || {};
 // punchOffset: themes whose impact is NOT the hero onset (e.g. flapboard's
 // lock-complete clack) shift the plate punch anchor; default keeps onset+2f
 const anchorT = (heroIn + (P.punchOffset ?? 0.045)).toFixed(3);
@@ -8562,15 +8562,16 @@ if (P.punch) {
 const rgba = P.rgbashift
   ? `,rgbashift=rh=-${P.rgbashift}:bh=${P.rgbashift}:enable='between(t,${anchorT},${(heroIn + 0.125).toFixed(3)})+between(t,${(LASTWORD.start + 0.04).toFixed(3)},${(LASTWORD.start + 0.17).toFixed(3)})',format=yuv420p`
   : "";
+const grain = P.grain ?? 5;
+const noise = grain === 0 ? "" : `,\n  noise=alls=${grain}:allf=t+u`;
 fs.writeFileSync(
   path.join(PROJECT, "_postfx.sh"),
   `#!/usr/bin/env bash
 # generated by make-theme.cjs — plate reaction for theme "${dna.name}"
 set -euo pipefail
 cd "$(dirname "$0")"
 ffmpeg -y -v error -i final.mp4 -filter_complex "
-  [0:v]${filter}${rgba},
-  noise=alls=${P.grain || 5}:allf=t+u[v]" \\
+  [0:v]${filter}${rgba}${noise}[v]" \\
   -map "[v]" -map 0:a -c:v libx264 -crf 14 -preset slow -profile:v high -c:a copy \\
   final_fx.mp4
 echo "[postfx] ${dna.name} → final_fx.mp4"

skills/embedded-captions/scripts/make-theme.test.mjs

@@ -0,0 +1,61 @@
+import assert from "node:assert/strict";
+import { execFileSync } from "node:child_process";
+import { cpSync, mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { dirname, join } from "node:path";
+import test from "node:test";
+import { fileURLToPath } from "node:url";
+
+const SCRIPT = fileURLToPath(new URL("./make-theme.cjs", import.meta.url));
+const SKILL = dirname(dirname(SCRIPT));
+
+function compilePostFx(grain) {
+  const root = mkdtempSync(join(tmpdir(), "embedded-captions-theme-"));
+  const skill = join(root, "skill");
+  const project = join(root, "project");
+
+  mkdirSync(join(skill, "scripts"), { recursive: true });
+  mkdirSync(join(skill, "themes"), { recursive: true });
+  mkdirSync(project);
+  cpSync(SCRIPT, join(skill, "scripts", "make-theme.cjs"), { recursive: true });
+  const dna = JSON.parse(readFileSync(join(SKILL, "themes", "anchor.json"), "utf8"));
+  if (grain === undefined) delete dna.plate.grain;
+  else dna.plate.grain = grain;
+  writeFileSync(join(skill, "themes", "anchor.json"), JSON.stringify(dna));
+
+  writeFileSync(
+    join(project, "theme.json"),
+    JSON.stringify({
+      dna: "anchor",
+      lines: [["HELLO"]],
+      width: 480,
+      height: 270,
+      fps: 24,
+      duration: 2,
+    }),
+  );
+  writeFileSync(
+    join(project, "transcript.json"),
+    JSON.stringify({ words: [{ text: "HELLO", start: 0.3, end: 0.7 }] }),
+  );
+
+  try {
+    execFileSync(process.execPath, [join(skill, "scripts", "make-theme.cjs"), project]);
+    return readFileSync(join(project, "_postfx.sh"), "utf8");
+  } finally {
+    rmSync(root, { recursive: true, force: true });
+  }
+}
+
+test("explicit plate grain zero omits the noise filter", () => {
+  const postFx = compilePostFx(0);
+
+  assert.doesNotMatch(postFx, /noise=alls=/);
+  assert.match(postFx, /\[0:v\]null\[v\]/);
+});
+
+test("missing plate grain retains the default noise level", () => {
+  const postFx = compilePostFx(undefined);
+
+  assert.match(postFx, /noise=alls=5:allf=t\+u/);
+});