.headroom's rules leave src/web/style/content.ts and its name leaves docs/DESIGN.md §5d #4405

Part two of #3834.

Done:

• [ ] git grep -n 'headroom' src/web finds no class use outside its rules.
• [ ] The rules at src/web/style/content.ts:272–278 leave, including the two narrow-screen .row:has(.headroom) rules, and .headroom leaves its §5d bullet in docs/DESIGN.md.
• [ ] Run bun scripts/design-lint.ts --update. Commit docs/design/baseline.json only if the count goes down or stays equal.
• [ ] bun x tsc --noEmit and bun test test/design-lint.test.ts pass.

Parked by the crew: the check stayed red through 2 automatic tries (pull #4441 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed ("strangers at every door > (unnamed)") and the decider saw no point in another try (pull #4460 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries (pull #4441 closed)
2. the check failed ("strangers at every door > (unnamed)") and the decider saw no point in another try (pull #4460 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Replaced by #4610. Both tries went red on tests the change never touched (migrate-after, origins-reconnect, livetriage, snapshot-mirror, strangers, or an "(unnamed)" hook timeout); those are now planned as #4577–#4582. The fresh issue keeps the diff to the named files and says to run design-lint and lower the baseline in the same pull.