Jay Moran's pin demos are on main: src/actions/pin_demos.ts, with launchOf in src/git/builds.ts #9272

First prerequisite of the display model (#9207 was parked twice because this file is not on main; #9194 closed without it).

Done looks like:

• [x] git checkout origin/incoming/jay-main -- src/actions/pin_demos.ts; his code as he wrote it, adapting only imports to main.
• [x] launchOf (and any other export pin_demos.ts imports from src/git/builds.ts that main lacks, e.g. readBuildPin) is added to main's src/git/builds.ts from his branch. Nothing main already exports there is removed or renamed.
• [x] Nothing in src/http or src/web is wired to it yet; it is reachable from a test.
• [x] test/pin-demos.test.ts: demoDefaults names a demo from a label and pages, demoFactsAt reads a pinned commit's facts, and saveDemo by a non-writer is refused with the access error. Each fails without the file.
• [x] bun x tsc --noEmit and bun test pass.
• [x] Commits carry Co-authored-by: Jay Moran.

Files: src/actions/pin_demos.ts, src/git/builds.ts, test/pin-demos.test.ts. Risks: builds.ts on his branch differs by ~190 lines; take only the missing exports, never the whole file.

Reviewed pull #9284 at its head: ticked git checkout origin/incoming/jay-main -- src/actions/pin_demos.ts; his code as he wrote it, adapting only imports to main.; launchOf (and any other export pin_demos.ts imports from src/git/builds.ts that main lacks, e.g. readBuildPin) is added to main's src/git/builds.ts from his branch. Nothing main already exports there is removed or renamed.; Nothing in src/http or src/web is wired to it yet; it is reachable from a test.; test/pin-demos.test.ts: demoDefaults names a demo from a label and pages, demoFactsAt reads a pinned commit's facts, and saveDemo by a non-writer is refused with the access error. Each fails without the file.; bun x tsc --noEmit and bun test pass.; Commits carry Co-authored-by: Jay Moran..