fix(#2805): slug-targeted image refresh honours plural slugs input (no whole-catalog fallback)

[jung-thomas]

Fixes #2805.

Root cause

Run 38816499609 was a workflow_dispatch with mode=slug-targeted and the plural slugs= input set (singular slug empty). The Refresh tutorial images (slug-targeted) step resolved its slug from only inputs.slug || dispatch_slug — it never read inputs.slugs — so REFRESH_SLUG was empty, backfill-images.ts --slug "" enumerated the whole catalog (21,712 images), and the step took 361s instead of the assumed 0–2s. Not a slow slug: a silent whole-catalog fallback whenever an operator uses the plural input.

The publish side of this same gap was already fixed on DEV (PUBLISH_SLUGS, filterToSlugs, dispatch_slugs); the image path was never updated. This PR mirrors that pattern onto the image path.

Changes

• scripts/backfill-images.ts — accepts a comma-separated list via --slug and/or --slugs (unioned, trimmed, blank-dropped, de-duped order-preserving); collectImageUrls takes a slug array and scans only those tutorials' built HTML. Empty/absent → whole-catalog (unchanged default). Unknown slug → empty map, no throw (best-effort warm). New unit tests: test/unit/backfill-images-slug-filter.test.js (9 cases).
• .github/workflows/rebuild-content.yml — the slug-targeted image step now also passes REFRESH_SLUGS: inputs.slugs || dispatch_slugs and invokes backfill-images.ts --slug "$REFRESH_SLUG" --slugs "$REFRESH_SLUGS", mirroring the publish step's PUBLISH_SLUGS. Also closes the same gap on the repository_dispatch multi-slug trigger.
• Docs — publish-performance-spike-2802.md Finding 3 records the root cause + fix; design + plan under docs/superpowers/.

Verification

• npx vitest run test/unit/backfill-images-slug-filter.test.js → 9/9 pass; no regressions in publish-content-slug-filter / publish-image-warm.
• Failing-run shape traced: --slug "" --slugs "a,b" now scopes to a,b (not whole-catalog, no crash); both-empty still enumerates the full catalog.
• CI proof (post-merge to DEV): dispatch rebuild-content.yml -f mode=slug-targeted -f slugs=<one-slug> and confirm the image step logs (slugs: …) + a small image count and runs in seconds, not 361s.

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