fix: preserve explicit zero caption grain

[miguel-heygen]

fixes reported:1785872503.756319; no related claim remains outstanding

Source claim: https://slack.com/archives/C0BGC335AQY/p1785872503756319

Summary:

• preserve an explicit plate grain value of zero instead of replacing it with the default
• omit the FFmpeg noise filter when resolved grain is zero
• retain noise=alls=5 when the grain setting is absent

Regression coverage:

• explicit grain zero compiles without a noise filter
• missing grain continues to compile with the default level

Verification:

• node --check skills/embedded-captions/scripts/make-theme.cjs
• node --test skills/embedded-captions/scripts/make-theme.test.mjs (2 passed)
• bun run test:skills (463 passed)
• bunx oxlint changed files
• bunx oxfmt --check changed files