Fast test/capacity.test.ts: chop and rewrite under the budget (1 s a test, 10 s a file) #1326

Part of #1242. test/capacity.test.ts took 15.9 s alone (spawn to exit) in the fast check run 01m4v94pk641ne5hzw5cntvwr1 on the gate box, with 2 tests over 1 s:

• the crew's concurrency and shard count reach the next pick > raised from one to two at boot's one, two agents run at once; each shell is told the shards — 7.76 s
• the next check honours a concurrency the admin lowered, and gets the shard count > lowered to one, three repositories' checks run one at a time; each sees FORGE_CHECK_SHARDS — 3.28 s

It sits in SLOW in scripts/gate.ts with this issue beside it, so the gate's time budget does not turn it red meanwhile.

Chop and rewrite, not trim (the owner, on #1242):

• Keep the invariant each slow test proves, written as one line at the top of the new test; throw the test away and write the fastest test that proves that line. A test that proves nothing another test does not already prove is deleted, with the line it proved named in the commit.
• One forge and one server per file, created in beforeAll; no Bun.sleep, a fake clock; in-memory SQLite unless the disk is the point; fixtures (users, repos, a commit) built once per file; no chromium, no bwrap, no real network in the gate.
• Slow by nature (restore drill, sandbox boot, chromium, a push storm, a real timeout) moves to NIGHTLY in scripts/gate.ts, with its reason.

Done: the gate's scripts/test-times.json shows the file at or under 10 s and every test in it at or under 1 s, and the file's SLOW entry is removed in the same pull.

Replaced by #10216: still over the 10 s budget when measured on 2026-11-09; re-planned under P0 now that #1163 is gone.