docs(extractor): explain why exported styled components keep selector markers

.changepacks/changepack_log_selector_markers_docs.json

@@ -0,0 +1,7 @@
+{
+  "changes": {
+    "bindings/devup-ui-wasm/package.json": "Patch"
+  },
+  "note": "The limitations page documents why every exported styled component carries a component-selector marker class even when no selector in its file names it: a module importing it may select or extend it, and each file is built without knowing its importers. No runtime changes or new build errors.",
+  "date": "2026-11-15T00:00:00.000Z"
+}

apps/landing/src/app/(detail)/docs/limitations/page.mdx

@@ -87,6 +87,7 @@ A dynamic value such as `width` becomes a CSS variable set in `style`, so a comp
 
 ## Known limitations
 
+- An exported styled component always carries one short marker class (`a--b`), even when no selector in its own file names it. A module importing it may select it (`${Child} { ... }`) or extend it, and each file is built without knowing which files import it, so the class has to be on the element from the start. A styled component the file keeps to itself carries a marker only when a selector names it.
 - A class, `keyframes` name or styled component defined in another module is not read as a build-time value: its name is only known once that file is built. Declare it in the file that uses it, or read it from a style prop.
 - `css()`, `globalCss()` and `keyframes()` have no element to set a CSS variable on, so a runtime value there is a build error. Move it to a style prop.
 - A styled-components or Emotion style function may read the theme only as `theme.a.b` (or `theme.a[2]`). Calls, whole-theme reads and runtime keys are build errors.

libs/extractor/src/visit.rs

@@ -1302,7 +1302,14 @@ impl<'a> DevupVisitor<'a> {
 
     /// The classes other styles select the component bound to `name` by: one
     /// for each name the file exports it as, and a class of its own when it
-    /// is only selected here
+    /// is only selected here.
+    ///
+    /// An exported component carries its markers even when nothing here
+    /// selects it: a module importing it may select it (`${Child} { ... }`,
+    /// `[Child]`) or extend it, naming the same marker from the export, and
+    /// each module is built without knowing which modules import it. Only
+    /// the element the component renders can carry the class, so the marker
+    /// cannot wait for an importer to ask for it.
     fn markers_of(&mut self, name: &str, exported: bool, selected: bool) -> Vec<(String, usize)> {
         let mut markers: Vec<(String, usize)> = if exported {
             self.exports