core: register built-in "light" theme

Add a small core-theme-builtin.c module containing the core
contribution to the "light" theme: 33 overrides for
"weechat.bar.{status,title}.color_*" and "weechat.color.*" tuned for
light-background terminals.

theme_builtin_init() builds a hashtable from the static entry table and
calls theme_register("light", overrides), then frees the temporary
hashtable. It is called once from weechat_init right after theme_init.
Calling it twice is a no-op (the registry merges identical keys).

Default option values are NOT changed. Existing configs render exactly
as before; users opt in with "/theme apply light".

Add TEST(CoreTheme, BuiltinInit) covering:
  - the "light" theme is absent before theme_builtin_init();
  - it is present after, with >= 30 overrides;
  - three spot-checked values match the source table;
  - calling theme_builtin_init() a second time does not change the
    override count.

Plugins contribute their own "light" overrides via weechat_theme_register
in subsequent commits.

src/core/core-theme.h

@@ -55,4 +55,7 @@ extern void theme_free (struct t_theme *theme);
 extern void theme_init (void);
 extern void theme_end (void);
 
+/* implemented in core-theme-builtin.c */
+extern void theme_builtin_init (void);
+
 #endif /* WEECHAT_THEME_H */