script: track per-script theme contributions and purge on script unload

Make individual scripts the unit of ownership for theme contributions
so that loading a script that calls weechat.theme_register(...) and
later unloading it correctly removes the script's overrides.

Plugin API addition (weechat-plugin.h):

  - new function pointer t_weechat_plugin.theme_unregister_script
    delegates to core's theme_unregister_script.
  - new convenience macro weechat_theme_unregister_script(script).
  - WEECHAT_PLUGIN_API_VERSION bumped to 20260527-02.

Script API additions (plugin-script-api.{c,h}):

  - new plugin_script_api_theme_register (plugin, script, name,
    overrides) forwards to the plugin API with the script pointer
    as the contribution owner, so contributions are tracked
    per-script (not per script-language plugin).

Lifecycle wiring (plugin-script.c):

  - new file-local plugin_script_remove_themes (plugin, script)
    calls weechat_theme_unregister_script.
  - plugin_script_remove now calls it alongside the other
    plugin_script_remove_* helpers, so script-unload tears down
    everything (configs, bar items, themes, hooks).

Eight language bindings updated to call
plugin_script_api_theme_register instead of weechat_theme_register
directly, so they pass the script pointer as owner:

  - python, perl, ruby, lua, tcl, javascript, php, guile.

The behavior change is end-to-end visible only at script unload:
before, an unloaded script's theme overrides lingered in the
registry forever and would be re-applied on the next /theme apply;
after, they vanish when the script unloads. /plugin unload of an
entire script-language plugin already worked via commit 24's hook
in plugin_remove (which drops both plugin-only contributions and
their script-owned children when the language plugin itself goes
away, because the contribution's plugin pointer matches).

No new unit tests in this commit: the underlying theme_unregister_script
function is covered by TEST(CoreTheme, UnregisterByOwner), and the
remaining changes are plumbing.

src/plugins/plugin-script-api.c

@@ -1568,6 +1568,28 @@ plugin_script_api_config_unset_plugin (struct t_weechat_plugin *weechat_plugin,
     return return_code;
 }
 
+/*
+ * Register theme overrides on behalf of a script.
+ *
+ * Forwards to weechat_plugin->theme_register with the script pointer
+ * as the owner, so the contribution can be auto-purged when the
+ * script unloads (see plugin_script_remove_themes).
+ */
+
+struct t_theme *
+plugin_script_api_theme_register (struct t_weechat_plugin *weechat_plugin,
+                                  struct t_plugin_script *script,
+                                  const char *name,
+                                  struct t_hashtable *overrides)
+{
+    if (!script)
+        return NULL;
+    return (weechat_plugin->theme_register) (weechat_plugin,
+                                             script,
+                                             name,
+                                             overrides);
+}
+
 /*
  * Create an upgrade file.
  *