fix(docs[conf]) chain gp-sphinx setup() to restore FOWT prevention

why: docs/conf.py defined `def setup(app)` AFTER
`globals().update(conf)`, which silently shadowed the `setup`
callback `gp_sphinx.config.merge_sphinx_config()` returns. With
gp-sphinx's setup never running, the inline `<script
data-cfasync="false">` that gates body paint until `data-theme` is
set was never injected — causing the dark→light flicker visible at
load time. Mirrors the pop+chain pattern at vcspull/docs/conf.py.

what:
- pop `conf["setup"]` into `_gp_setup` before `globals().update(conf)`
- call `_gp_setup(app)` as the first statement of the user's
  `setup(app)` so gp-sphinx's hooks register first

side effects (all previously silently disabled, now restored):
- `js/spa-nav.js` SPA-navigation script
- GP_SPHINX_COPYBUTTON_SELECTOR bridge for sphinx-copybutton
- `remove_tabs_js` build-finished cleanup
- `myst` / `myst-md` Pygments lexers

Author: tony

docs/conf.py

@@ -62,6 +62,8 @@
     html_extra_path=["manifest.json"],
     rediraffe_redirects="redirects.txt",
 )
+
+_gp_setup = conf.pop("setup")
 globals().update(conf)
 
 
@@ -91,5 +93,14 @@ def _on_missing_class_reference(
 
 
 def setup(app: Sphinx) -> None:
-    """Connect missing-reference handler to resolve py:data as :class: links."""
+    """Chain gp-sphinx setup() and connect the missing-reference handler.
+
+    ``_gp_setup`` is the callback ``gp_sphinx.config.merge_sphinx_config``
+    returns; it registers the FOWT-prevention head injection,
+    ``spa-nav.js``, the copy-button bridge, the ``remove_tabs_js``
+    cleanup, and the MyST Pygments lexers. Without this chain those
+    hooks never register and the docs site flashes the wrong theme
+    before settling on the user's preference.
+    """
+    _gp_setup(app)
     app.connect("missing-reference", _on_missing_class_reference)