merge: bring the element-style work onto main's api_v3 package split

main moved 16 commits while this branch was open, and one of them split
api_v3.py into a package. A rebase would have conflicted on that file in
each of the 15 commits that touch it; merging resolves it once.

Reapplied into the package, unchanged in substance:

- _schema_type_is / _schema_allows_null and the nullable-field handling
  (api_v3/__init__.py, where the form parser now lives)
- the indexed-array recombiner's union-type fix and its all-blank-means-null
  case (api_v3/plugins.py)
- scalable / native_size on the font catalog, and the removal of the three
  font-override stubs (api_v3/fonts.py)

Two places took main's version rather than mine, deliberately:

- pages_v3 now reads the schema through SchemaManager *and* keeps
  resolve_under for the raw-read fallback, so the containment guard main
  added in the CodeQL triage survives.
- serve_plugin_widget moves off os.path.basename onto safe_path_component
  and resolve_under. main dropped `import os` from pages_v3 in favour of
  those helpers, and they reject a bad component rather than mangling it
  into a valid-looking one.

The url-map snapshot loses exactly the three font-override routes. Edited
rather than regenerated: that test exists to catch a route lost by accident
during a move, and regenerating it wholesale is how such a loss would be
papered over.

Verified against a fresh run of current main: same failure set, 4692
passing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/plugin_system/base_plugin.py

@@ -684,6 +684,38 @@ class BasePlugin(ABC):
         """
         return
 
+    def get_update_interval(self) -> Optional[float]:
+        """
+        How often this plugin wants update() called, right now, in seconds.
+
+        The manifest's ``update_interval`` is a single static number, which
+        cannot say "poll me every 15 seconds while a game is in progress and
+        every 15 minutes when nothing is on". Only the plugin knows which is
+        true at any moment, so override this to say so.
+
+        Return None (the default) to accept the manifest/config value.
+
+        Two constraints, both because the scheduler calls this on every tick of
+        the render loop:
+
+        - It must be cheap. Attribute reads only -- no config lookups, no I/O,
+          no locks that a fetch might be holding.
+        - It must not raise. A raising hook is ignored and the static interval
+          used, but a hook that raises every tick also logs every tick.
+
+        Values below PluginManager.MIN_DYNAMIC_UPDATE_INTERVAL are clamped up:
+        a plugin asking for 0 would otherwise busy-wait against its own API.
+
+        Example::
+
+            def get_update_interval(self):
+                # Fast while something is actually live, manifest default otherwise.
+                if any(m.live_games for m in self._live_managers):
+                    return self.config.get("live_update_interval", 15)
+                return None
+        """
+        return None
+
     def has_live_priority(self) -> bool:
         """
         Check if this plugin has live priority enabled.