Add check:placeholders — enforce the card placeholder warning

[Mimmo2026]

What

Adds npm run check:placeholders, a publish gate for the placeholder content the kit's own docs already warn about:

Library names, handles, statistics, and source labels inside cards are illustrative placeholders. Replace them with your own verified copy before publishing a video.

Nothing currently enforces that. Ship an unreplaced card and you publish an invented statistic under a fabricated source line, on your own channel.

Scale

Running it against the shipped library finds 135 fail-level placeholders across the 406 cards:

Rule	Count
source-label — fabricated SOURCE · … 2026 citation lines	125
placeholder-handle — @handle, @yourchannel	6
font-placeholder — "FontName"	3
shipped-handle — a specific handle carried in the cards	1

The citation lines are the dangerous ones: they attach a source to a number that has no source.

Design

• Scans video-projects/ only, never the style library. The library is supposed to contain placeholders; failing on it would fail always.
• Statistics warn, they do not fail. 73%, 58B, 4x get surfaced for a human to check, because the script cannot know which are legitimately the author's.
• Exit 1 on any fail-level match, so it drops into a pre-publish step or CI.

Usage

npm run check:placeholders              # scan video-projects/
npm run check:placeholders -- --list    # show every rule and why it matters
node scripts/check-placeholders.mjs path/to/project

No new dependencies. npm run check and npm test both still pass.