feat(cli): --frame-check accepts a severity/seek/tol spec

The pipeline already carried FrameCheckOptions; only the flag was
boolean, which meant a pipeline caller tuning severity or seek points
would have them silently dropped — the two sides only agreed because
today's caller happens to match the defaults. Bare --frame-check keeps
the defaults; the value form mirrors --caption-zone's grammar, freezing
the contract before a release pins it.

packages/cli/src/commands/check.test.ts

@@ -1112,6 +1112,22 @@ describe("check pipeline", () => {
   });
 });
 
+describe("frame-check flag grammar", () => {
+  it("keeps bare --frame-check on defaults and parses the value form", async () => {
+    const { parseFrameCheck } = await import("./check.js");
+    expect(parseFrameCheck(undefined)).toBeUndefined();
+    expect(parseFrameCheck(true)).toEqual({});
+    expect(parseFrameCheck("")).toEqual({});
+    expect(parseFrameCheck("severity=error;seek=.25,.75;tol=4")).toEqual({
+      severity: "error",
+      seek: [0.25, 0.75],
+      tol: 4,
+    });
+    expect(() => parseFrameCheck("bogus=1")).toThrow("Invalid --frame-check");
+    expect(() => parseFrameCheck("tol=-2")).toThrow("Invalid --frame-check");
+  });
+});
+
 describe("contrast persistence", () => {
   it("demotes a single-sample contrast failure to warning but gates held failures", async () => {
     const driver = fakeDriver({