A check's bun install fails with registry 429: the egress cap is 8 connections, bun opens 48 #3983

Every check on pull #3926 (and any pull without a warm install) fails in bun install before a test runs:

egress capped: check:… already has 8 connections to registry.npmjs.org:443
error: GET https://registry.npmjs.org/@types/node/-/node-26.1.2.tgz - 429

Cause: src/apps/egress.ts refuses a cell more than 8 open connections to one destination (added with #3721); bun install defaults to --network-concurrency 48, and bun reports the refusals as 429s.

Done:

• [ ] A check's default install runs within the cap (for example bun install --network-concurrency 4 in the default install command in src/apps/manifest.ts), or the cap lets the registry through; a test covers whichever.
• [ ] A pull with no cached install passes its check.

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.