feat(cli): snapshot --zoom and per-finding crops on check --snapshots

snapshot --zoom <selector|x,y,w,h> + --zoom-scale (default 3) crops via
Puppeteer clip at raised deviceScaleFactor — density changes, layout
never does. Selector resolves per frame with 24px padding; no match is
a loud error, and a frame whose clamped region is a sliver (element
collapsed or animated off-canvas) is skipped with a stderr note rather
than written as a useless few-pixel image.

check --snapshots additionally writes finding-NN-<code>.png crops for
error findings with bboxes (cap 12, deterministic re-seek in a second
session) and draws labeled annotation boxes on overview frames via a
transient overlay injected only after audits complete. Skill reference
gains the zoom workflow: check reports a finding, zoom into it, fix,
re-check.

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

@@ -1,5 +1,9 @@
 import { describe, expect, it } from "vitest";
-import { computeSnapshotTimes, tailFrameTime } from "./snapshot.js";
+import { computeSnapshotTimes, parseZoomScale, tailFrameTime } from "./snapshot.js";
+
+// --zoom's crop-region math (selector bbox + padding + clamp, exact region
+// form, no-match error) is owned by and tested in
+// ../capture/captureCompositionFrame.test.ts alongside its implementation.
 
 describe("tailFrameTime", () => {
   it("backs off ~3% of duration so the final frame isn't the blank exact-end", () => {
@@ -59,3 +63,19 @@ describe("computeSnapshotTimes (FINDING [7]: tail is always captured)", () => {
     expect(appendedTail).toBe(false);
   });
 });
+
+describe("parseZoomScale (--zoom-scale)", () => {
+  it("defaults to 3 when unset", () => {
+    expect(parseZoomScale(undefined)).toBe(3);
+  });
+
+  it("honors an explicit scale", () => {
+    expect(parseZoomScale("2")).toBe(2);
+  });
+
+  it("falls back to the default for invalid or non-positive input", () => {
+    expect(parseZoomScale("abc")).toBe(3);
+    expect(parseZoomScale("0")).toBe(3);
+    expect(parseZoomScale("-1")).toBe(3);
+  });
+});