Spec explorer data endpoint, spec file finder, browser render test (failing)

- Add spec-explorer-data-by-slug helper with _SPEC_SLUG_MAP
- _find_spec_file searches spec/, web/, shared/sx/ref/ directories
- defpage specs-explore-page uses :data for server-side data fetch
- test_evaluator_renders_in_browser: failing test for client-side rendering
  (client re-evaluates defpage content, find-spec unavailable — pre-existing)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/tests/test_demos.py

@@ -491,6 +491,15 @@ class TestSpecExplorer:
         assert "define" in body, "Should contain define forms from spec"
         assert "eval-expr" in body, "Should contain eval-expr from evaluator spec"
 
+    def test_evaluator_renders_in_browser(self, page: Page):
+        """Spec explorer should render correctly in the browser, not show 'not found'."""
+        nav(page, "(language.(spec.(explore.evaluator)))")
+        page.wait_for_timeout(3000)
+        content = page.locator("#main-panel").text_content() or ""
+        assert "not found" not in content.lower(), \
+            f"Page shows 'not found' instead of spec content: {content[:200]}"
+        expect(page.locator("#main-panel")).to_contain_text("Evaluator", timeout=5000)
+
 
 # ---------------------------------------------------------------------------
 # Key doc pages (smoke tests)