fix(design): the button and chip hover colours were never generated — an act: variant instead of a runtime prefix (E28)

buttonStyles/chipStyles glued the hover guard on at runtime (`${ACT}hover:text-fg`). Tailwind emits CSS
only for a class it finds whole in the source, so all 22 guarded hovers shipped with no rule: no text or
edge colour on hover for ghost/outline/danger buttons, no white layer on the player's control row, no
underline on a link button, no accent hover on token/suggest/ghost chips. The stylesheet had 0 rules
for `not-aria-disabled`.

- `@custom-variant act` (index.css): the same guard as the state layers; `:disabled` never matches an
  <a>, so one variant serves buttons and links and the per-element variant maps go.
- ui/tailwindScan.contract.test.ts: every class the primitives can emit stands whole in their source.
- a lint rule refuses a variant right after an interpolation; the `enabled:hover:` rule now points at
  `act:hover:`.

frontend/src/lib/controlRules.contract.test.ts

@@ -18,7 +18,7 @@ describe("a native select", () => {
   });
 });
 
-// `enabled:hover:` is blind to a focused FocusSafeButton (aria-disabled, not the native attribute),
+// `enabled:hover:` is blind to a focused FocusSafeButton (aria-disabled, not the native attribute; `act:` is not),
 // which then lit up on hover as if a click would work.
 describe("a hover gated on `enabled:` alone", () => {
   it("is refused; the aria-aware gate is not", async () => {
@@ -33,6 +33,28 @@ describe("a hover gated on `enabled:` alone", () => {
         )
       )[0]?.messages.filter((m) => m.message.startsWith("`enabled:hover:`")).length;
     expect(await lint("enabled:hover")).toBe(1);
-    expect(await lint("enabled:not-aria-disabled:hover")).toBe(0);
+    expect(await lint("act:hover")).toBe(0);
+  });
+});
+
+// A variant glued onto an interpolated prefix is never seen whole by Tailwind's scanner, so it ships
+// with no CSS (every button and chip hover colour died that way). A plain interpolated value is fine.
+describe("a class split by an interpolation", () => {
+  const split = async (src: string) =>
+    (await eslint.lintText(src, { filePath: "src/components/__probe.tsx" }))[0]?.messages.filter(
+      (m) => m.message.startsWith("a class split by an interpolation"),
+    ).length;
+  const act = "enabled:"; // assembled, as the offender was
+  it("is refused after an expression", async () => {
+    expect(await split("const A = 'x';\nexport const b = `${A}" + "hover:text-fg`;\n")).toBe(1);
+    expect(await split("const A = 'x';\nexport const c = `p-1 ${A}" + act + "hover:x`;\n")).toBe(1);
+  });
+  it("leaves a whole class, a leading variant and a plain value alone", async () => {
+    expect(await split("const n = 1;\nexport const d = `hover:text-fg w-${n} ${n}px`;\n")).toBe(0);
+    expect(
+      await split(
+        "const u = 'h';\nexport const e = `${u}://x:${n()}`;\nfunction n() { return 1; }\n",
+      ),
+    ).toBe(0);
   });
 });