fix: preserve explicit zero caption grain - #3032

Clean, correct fix.

The bug: || coerces 0 to falsy, so (dna.plate || {}).grain || 5 replaced an explicit grain: 0 with 5 — the noise filter always ran even when the theme author explicitly disabled it.

The fix: ?? preserves 0 (only null/undefined trigger the default). The noise filter string is now built conditionally — empty when grain === 0, which cleanly removes the ,noise=alls=... segment from the ffmpeg filter graph rather than passing noise=alls=0 (which would still allocate the filter for no visual effect).

Two things I verified:

• The filter + rgba + noise concatenation still produces a valid filter graph in both the zero and non-zero paths. The null filter name in the test output ([0:v]null[v]) is expected — it's the upstream filter variable's value when no spatial filter is active.
• The P.grain || 5 on line 8540 (heroless branch) is also replaced with ??, so the grain value is consistent between the plate object construction and its later use.

Test coverage is proportionate: explicit-zero and missing-grain are the two boundary cases that matter. No notes.

— Miga

No blocking issues found in my review. Stamping. — Miga