notice content changes and rebuild without a restart

Polling lives in internal/ext/watch, per the human's call to keep core under its
ceiling rather than raise it a second time — which is what ADR-0041 said a second
raise would mean. It is a poller, deletable without trace, and core stayed at
2671/2800.

A settled change calls the same `rebuilder` that startup calls, because a reload
path that differs from the startup path is a reload path that drifts. The index is
an atomic.Pointer swapped whole, so a request reads the site that was current when
it arrived instead of one being rebuilt underneath it — the alternative, mutating in
place, is a data race with every in-flight request.

Names, sizes and modification times, not contents: reading every file to detect a
change costs more than the rebuild it triggers. Editor droppings are excluded,
because saving in vim writes a swap file, a backup and the number 4913, and each
would otherwise look like a change. A change must hold still for a moment first,
since one save is often several operations.

Verified against the running binary: a page 404s, the file appears, and five seconds
later it serves — one "site root changed" in the log. Then three droppings written
at once produced no rebuild at all.

Two warnings fired and were fixed rather than silenced: `runServe` gave up the
rebuild closure to `rebuilder`, and the fingerprint walk gave up its body to
`record`, where three exclusions read as a list instead of as nesting.

The Dockerfile ships the binary alone. The site root arrives as a volume and is
never copied in — it is somebody's content repository with its own history
(ADR-0011), so the image is the same for every site.

internal/web/asset_test.go

@@ -38,7 +38,7 @@ func assetHandler(t *testing.T) http.Handler {
 	if err != nil {
 		t.Fatal(err)
 	}
-	return Handler(content.NewSite(bundles), r, fsys, nil, content.Settings{})
+	return Handler(Fixed(content.NewSite(bundles)), r, fsys, nil, content.Settings{})
 }
 
 func TestABundlesOwnFilesAreServed(t *testing.T) {