fix(diary): retire replaced daily writers and serialise ApplyGlobals

- install and Reset retire the channels they replace: a retired daily
  writer appends a late record and closes the file again, so no record is
  lost and no descriptor stays open; diary.Close keeps reopening the
  installed writers on the next record
- ApplyGlobals holds the setup lock for its whole run (lock-free
  installLocked and setVerbosityLocked), so concurrent calls rebuild the
  channels once (WR-06)

modules/diary/diary.go

@@ -156,6 +156,12 @@ func Setup(cfg *compass.Config, opts Options) error {
 func install(conf Config, ch *channels, opts Options) {
 	mu.Lock()
 	defer mu.Unlock()
+	installLocked(conf, ch, opts)
+}
+
+// installLocked makes ch the installed setup and retires the channels it
+// replaces; the caller holds mu.
+func installLocked(conf Config, ch *channels, opts Options) {
 	if saved == nil {
 		saved = &savedDefault{logger: slog.Default(), writer: log.Writer(), flags: log.Flags()}
 	}
@@ -174,7 +180,7 @@ func install(conf Config, ch *channels, opts Options) {
 	old := current.Swap(st)
 	slog.SetDefault(slog.New(newComponentHandler("")))
 	if old != nil {
-		closeChannels(old.channels)
+		retireChannels(old.channels)
 	}
 }
 
@@ -186,6 +192,11 @@ func install(conf Config, ch *channels, opts Options) {
 func SetVerbosity(v int, quiet bool) {
 	mu.Lock()
 	defer mu.Unlock()
+	setVerbosityLocked(v, quiet)
+}
+
+// setVerbosityLocked is SetVerbosity for a caller that holds mu.
+func setVerbosityLocked(v int, quiet bool) {
 	verbosity.Store(int64(v))
 	st := current.Load()
 	if st == nil {
@@ -212,18 +223,21 @@ func FromGlobals(g bonfire.Globals) Options {
 // the verbosity and quiet switch (SetVerbosity) and, when it differs from
 // the one the channels were built with, the console color mode, which
 // rebuilds the channels from the same configuration. The generated main
-// registers it with bonfire.OnGlobals.
+// registers it with bonfire.OnGlobals. It holds the setup lock for its
+// whole run, so concurrent calls rebuild the channels once.
 func ApplyGlobals(g bonfire.Globals) {
+	mu.Lock()
+	defer mu.Unlock()
 	st := current.Load()
 	if st != nil && st.opts.Color != g.Color {
 		opts := st.opts
 		opts.Color, opts.Verbosity, opts.Quiet = g.Color, g.Verbosity, g.Quiet
 		if ch, err := buildChannels(st.conf, opts); err == nil {
-			install(st.conf, ch, opts)
+			installLocked(st.conf, ch, opts)
 			return
 		}
 	}
-	SetVerbosity(g.Verbosity, g.Quiet)
+	setVerbosityLocked(g.Verbosity, g.Quiet)
 }
 
 type hideOnTTYKey struct{}
@@ -264,6 +278,22 @@ func Close() error {
 	return closeChannels(st.channels)
 }
 
+// retireChannels closes the files of channels a new setup replaced, for
+// good: a goroutine that still holds the old state appends its record and
+// the file is closed again, so no descriptor stays open.
+func retireChannels(ch *channels) {
+	if ch == nil {
+		return
+	}
+	for _, c := range ch.closers {
+		if r, ok := c.(interface{ retire() error }); ok {
+			_ = r.retire()
+			continue
+		}
+		_ = c.Close()
+	}
+}
+
 func closeChannels(ch *channels) error {
 	if ch == nil {
 		return nil
@@ -275,9 +305,9 @@ func closeChannels(ch *channels) error {
 	return errors.Join(errs...)
 }
 
-// Reset drops the installed setup, closes its files and restores the process
-// default logger (and the standard log package's output) captured at the
-// first Bootstrap or Setup. Tests call it in t.Cleanup.
+// Reset drops the installed setup, closes its files for good and restores
+// the process default logger (and the standard log package's output)
+// captured at the first Bootstrap or Setup. Tests call it in t.Cleanup.
 func Reset() {
 	mu.Lock()
 	defer mu.Unlock()
@@ -290,6 +320,6 @@ func Reset() {
 		saved = nil
 	}
 	if old != nil {
-		closeChannels(old.channels)
+		retireChannels(old.channels)
 	}
 }