Justify the handlers tests' start limit with a measurement (closes #225)

The cost that pushed internal/handlers near its 90s package timeout, and test app starts past fx's 15s start timeout, was every start hashing the admin password at 64 MB; that went with #404. Measured since under the host's real load, the package takes 8.5s and the slowest test app start 0.49s, so neither limit changes. newTestApp now says what its start limit catches and the measurement behind it, and script/test notes that its figures predate that change and gives the current ones. No test changes what it asserts.

Model: opus-5-5

internal/handlers/handlers_test.go

@@ -181,6 +181,11 @@ func (r *recordingArchives) Renames() []archiveRename {
 	return out
 }
 
+// newTestApp returns an app whose RequireStart fails the test when
+// starting takes longer than fx's default start timeout of 15s. That
+// limit catches a start that hangs, not a busy host: measured with make
+// test on 2026-10-02 at host load 58-69 on 48 cores, the slowest of this
+// package's starts took 0.49s.
 func newTestApp(
 	t *testing.T,
 	targets ...any,