media: extract the shared rendiv kit

[simontaurus]

media/kit holds the rendiv mechanics the explainer, the tutorial series and the Prototype Fund pitch deck all repeat. The pitch deck lives in the private project-management repo and already pins this one for the design system, so it pulls the kit through the same pin instead of keeping a third copy.

Bugs this fixes

• Tutorials never patched their own node_modules. postinstall ran ../explainer/scripts/patch-rendiv.mjs, which resolved its root relative to the script, so it patched the explainer's install. Latent until a checkout path contains a backslash escape; media/tutorials under a path with \t fails every render with Rollup failed to resolve import "C:<TAB>mp...". patch-rendiv now resolves the root from the working directory, which is what npm sets for a lifecycle script.
• --theme and --suffix ate the first positional filter. Both callers open-coded argv.indexOf(flag) and skipped index i + 1; with the flag absent i is -1, so i + 1 is 0 and the first filter was dropped. build-all.mjs ep1 silently rendered nothing. Now one parseArgs.
• render-series.mjs hardcoded ffmpeg.exe, so it could not run on a Linux runner.

Segmented rendering

renderSegmented renders each segment as its own composition and concatenates with -c copy, so editing one segment costs one segment's render.

It is only sound when segments hard-cut. A cross-dissolve composites the outgoing scene under the incoming one; rendered alone, a segment's entry fade resolves against a transparent canvas and encodes as black. Measured on the pitch deck: a black flash at all 8 boundaries and no dissolve anywhere, with both the render and the concat reporting success. So renderSegmented refuses a non-zero overlap and assertNoBlackFrames checks the stitched file.

The explainer and the tutorial episodes cross-dissolve, so both keep rendering the whole timeline in one pass; their behaviour is unchanged.

Verified

• npm run stills -- 04-clarity (explainer), npm run sheet
• npm run stills -- --theme light ep1 (tutorials), 18 stills, after the patch fix
• npm run typecheck in both projects
• parseArgs against the four argument shapes the callers use

[github-actions]

Explainer frames

Beats, in reading order

1. 01-hook-panels
2. 02-hook-broken
3. 03-hook-punch
4. 04-clarity
5. 05-merge-apart
6. 06-merge-union
7. 07-code-reveal
8. 08-code-wash
9. 09-outputs
10. 10-domains
11. 11-domains-closing
12. 12-proof-story
13. 13-proof-facts
14. 14-end-card

Rendered from cf51d634a3801df8536f2a8da2baab196e77d000. Images live on the ci-review-images prerelease, not in the repository.