feat(design): the panel now asks whether the app agrees with its own sheet

Retiring rulebook #2 left the /design drift panel with no data source, and
because its empty state was well-written the feature read as working while it
could only ever render "nothing designated" (#2419). The original question is
genuinely gone: theme.css is generated from design system 2, so checking the
system against a sheet derived from it would be a tautology.

The question that survives is the one no server can answer. A generated sheet
still has to be LOADED and APPLIED, and nothing checked that it was:

  absent      the record declares a token the app doesn't have — the sheet was
              never regenerated after the record changed, or never loaded
  differs     the app has it with another value — a stale sheet, or a later
              rule that overrode it
  unrecorded  the app declares a token in the record's own family that the
              record has never heard of

Both sides go through the same engine so the comparison is honest: declared
values are set on an offscreen probe and read back, which performs the same
var() substitution the browser already did to the live values. Comparing raw
strings would mark every derived token as drift.

The designation moved with the feature — design_rulebook_id becomes
ui_design_system_id, with a migration deleting the retired key rather than
leaving an inert row. The prose extractor it fed goes too (#2288 said its
runtime role ended when the import landed).

Three orphans of the same shape, found alongside and fixed here:

- darkOverriddenNames hardcoded [data-theme="dark"]. The sheet went dark-first
  months ago, so it matched nothing and the "mode-aware" flag silently left the
  gallery. Now matches the SHAPE of a mode selector, which also holds for an
  install whose modes aren't light and dark.
- groupFor's prefix table never heard of --fs-, so 110 tokens sat under
  "other". Groups now come from the record where there is one; the table can
  only know families that shipped with the product (rule #115).
- The type scale was a hand-written table of nine sizes marked "no token",
  true when written and false since the scale was recorded. Now rendered from
  whatever size tokens the sheet declares, so it can't go stale twice.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

tests/test_routes_design.py

@@ -0,0 +1,55 @@
+"""The /api/design blueprint — this install's UI, not the design-system record.
+
+Modelled on tests/test_routes_design_systems.py. The URL enumeration is
+deliberate rather than a pattern match: this blueprint was repointed from
+`/expectations` to `/ui-system` (#2419), and the failure worth catching is the
+old rule surviving the change — a route nothing serves any more, answering with
+whatever the last handler registered on it did.
+"""
+import inspect
+
+import pytest
+
+
+def test_design_blueprint_registered():
+    from scribe.routes.design import design_bp
+    assert design_bp.name == "design"
+    assert design_bp.url_prefix == "/api/design"
+
+
+def test_design_blueprint_registered_in_app():
+    from scribe.app import create_app
+    app = create_app()
+    assert "design" in app.blueprints
+
+
+def test_the_blueprint_serves_exactly_one_rule():
+    from scribe.app import create_app
+    app = create_app()
+    rules = {
+        str(r.rule) for r in app.url_map.iter_rules()
+        if r.endpoint.startswith("design.")
+    }
+    assert rules == {"/api/design/ui-system"}
+
+
+def test_the_designation_reader_takes_user_id():
+    """The setting is per-user, so the read must be too (rule #78). A
+    module-level or admin-scoped read would hand one user another's designation."""
+    from scribe.services import design_systems as svc
+    assert "user_id" in inspect.signature(svc.ui_design_system).parameters
+
+
+def test_the_setting_key_is_the_new_one():
+    """Named explicitly because the panel silently reading a retired key is the
+    exact shape of the bug this replaced: a feature that renders, and reports
+    nothing, forever."""
+    from scribe.services.design_systems import UI_DESIGN_SYSTEM_SETTING
+    assert UI_DESIGN_SYSTEM_SETTING == "ui_design_system_id"
+
+
+def test_the_rulebook_expectation_extractor_is_gone():
+    """It was retired with the rulebook it parsed (#2288, #2419). Left importable
+    it would keep looking like a live path to the next reader."""
+    with pytest.raises(ModuleNotFoundError):
+        import scribe.services.design_rulebook_import  # noqa: F401