R-578: settings-callback deadlock gate over every controller package

TestR578NothingTakesTheSettingsLockInsideASettingsCallback (internal/settings) derives from
source the *Settings methods that take s.mu, the callback runners among them, and per package
every helper that transitively reaches one; a call to any of those inside a func literal
passed to a runner (or a non-literal callback) fails with file:line. Decoy:
TestR578GateConvictsAHelperChain. REUSE.md lookalike row added.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/backup/offsite_diag_test.go

@@ -203,6 +203,9 @@ func TestR570SentenceStaysHungarian(t *testing.T) {
 //
 // RED-PROOF (REPORT): put `m.note(...)` back inside the final UpdateOffboxStatus callback → this test
 // names the file and the line, in a second, instead of the suite hanging for 25 minutes.
+//
+// R-578: the controller-wide successor is TestR578NothingTakesTheSettingsLockInsideASettingsCallback
+// (internal/settings) — it derives the lockers and helpers from source in every package.
 func TestNoteHelpersAreNotCalledUnderTheSettingsLock(t *testing.T) {
 	callback := regexp.MustCompile(`\.Update\w*\(func\(`)
 	note := regexp.MustCompile(`\b(m|s)\.(note|noteErr|boxLang)\(`)