apply a template edit without a restart

The watcher fingerprinted templates/ but a rebuild only re-scanned content, so
editing a template fired a rebuild that changed nothing. ADR-0022 already
promised the opposite — "a template edit in the site root invalidates through
the same path as content" — which makes this a defect against a recorded
decision rather than a missing feature. ADR-0055 records the fix and supersedes
ADR-0048's narrower clause.

The parsed sets and the stylesheet become one parsedTheme behind an
atomic.Pointer, swapped by Refresh once per rebuild instead of per request. A
parse failure keeps the theme that was working, so a typo cannot take the site
down. The swap also retires the in-place field mutation -dev was doing, which
was a data race with every in-flight render.

site.yaml goes the other way and leaves the fingerprint: the settings are copied
by value into the renderer, the handler, the feeds and the sitemap, so applying
a change to some of them is worse than applying it to none. It is restart-only.

Corrects the Effects counter row while proving it did not move: it still said
startup was the only change signal "until queue 21", but queue 21 shipped as
ADR-0048 and put the derivative pass inside rebuilder, so that has been wrong
since. The row now also answers the question ADR-0055 invites — an in-memory
swap is not an Effect, because it writes no artifact and calls nothing outbound.

Measured on the real binary: a template edit went live in ~2s; a typo logged
"keeping the previous theme" and kept answering 200 with the last good markup; a
site.yaml edit now fires no rebuild at all. core 2766/2800, ext 1030/2000,
34 gates green, 0 warnings.

internal/ext/watch/watch.go

@@ -72,6 +72,10 @@ func Watch(fsys fs.FS, stop <-chan struct{}, onChange Changed) {
 // Names, sizes and modification times — not contents: reading every file to detect a change would cost more
 // than the rebuild it triggers. Editor droppings are excluded, or saving a file in vim would look like three
 // changes and a deletion.
+//
+// `content/` and `templates/` only. `site.yaml` is deliberately absent: a rebuild cannot apply it, since the
+// settings are copied by value into the renderer, the handler, the feeds and the sitemap, and a fingerprint
+// that fires a rebuild changing nothing is a lie told every two seconds (ADR-0055).
 func Fingerprint(fsys fs.FS) string {
 	sum := sha256.New()
 	for _, root := range []string{"content", "templates"} {
@@ -79,11 +83,6 @@ func Fingerprint(fsys fs.FS) string {
 			return record(sum, p, d, err)
 		})
 	}
-	// site.yaml is part of the site's state, and editing it should not need a restart.
-	if info, err := fs.Stat(fsys, "site.yaml"); err == nil {
-		sum.Write([]byte("site.yaml"))
-		_ = binary.Write(sum, binary.LittleEndian, info.ModTime().UnixNano())
-	}
 	return hex.EncodeToString(sum.Sum(nil))
 }
 

internal/ext/watch/watch_test.go

@@ -25,7 +25,6 @@ func TestFingerprintChangesOnlyForContent(t *testing.T) {
 		"edited a bundle":   {"content/posts/one.md": {Data: []byte("xx"), ModTime: time.Unix(200, 0)}},
 		"added a bundle":    {"content/posts/two.md": {Data: []byte("z"), ModTime: time.Unix(100, 0)}},
 		"edited a template": {"templates/page.html": {Data: []byte("yy"), ModTime: time.Unix(200, 0)}},
-		"edited site.yaml":  {"site.yaml": {Data: []byte("base: y"), ModTime: time.Unix(200, 0)}},
 	} {
 		next := fstest.MapFS{}
 		for k, v := range base {
@@ -39,6 +38,18 @@ func TestFingerprintChangesOnlyForContent(t *testing.T) {
 		}
 	}
 
+	// site.yaml is deliberately unwatched: a rebuild cannot apply it, because the settings are copied by value
+	// into the renderer, the handler, the feeds and the sitemap. Noticing it would fire a rebuild that changes
+	// nothing (ADR-0055), so editing it needs a restart.
+	settings := fstest.MapFS{}
+	for k, v := range base {
+		settings[k] = v
+	}
+	settings["site.yaml"] = &fstest.MapFile{Data: []byte("base: y"), ModTime: time.Unix(200, 0)}
+	if Fingerprint(settings) != before {
+		t.Error("site.yaml changed the fingerprint, so a rebuild will fire and apply nothing")
+	}
+
 	// Editor droppings are not content: saving in vim writes several of these, and each would look like a change.
 	noise := fstest.MapFS{}
 	for k, v := range base {