refactor(cli): single-source-of-truth pass over the check branch

Every duplicated decision gets one owner: rectToBbox lives in checkTypes
(was verbatim in pipeline and browser layers); the audit seek tuning is
one exported AUDIT_SEEK_OPTIONS consumed by check and the deprecated
inspect path; zoom padding/scale defaults export from the capture module
instead of re-literalized in three files; the optional run_id property
is built by one helper across all three telemetry events; check's
--max-transition-samples parsing reuses its own positiveInteger helper;
validate drops a leftover re-export and redundant explicit-default args
go away.

Tests: the contrast candidate round-trip gains a real integration
anchor (the actual browser script eval'd in-page, a wrapper asserting
finish receives the page-script bbox shape) replacing regex-over-source
as the primary guard; the redundant geometry source-golden and a
duplicated deprecation-envelope assertion are dropped.

packages/cli/src/utils/checkPipeline.ts

@@ -28,6 +28,7 @@ import {
   suggestCompliantForegroundColor,
   type Rgb,
 } from "../commands/contrast-bg.js";
+import { rectToBbox } from "./checkTypes.js";
 import type {
   AnchoredLayoutIssue,
   CheckAnnotationBox,
@@ -877,10 +878,6 @@ function failureReport(options: CheckOptions, finding: CheckFinding): CheckRepor
   return buildReport(options, lint, browser, { kind: "none" }, [], []);
 }
 
-function rectToBbox(rect: LayoutRect): CheckBbox {
-  return { x: rect.left, y: rect.top, width: rect.width, height: rect.height };
-}
-
 function isBbox(value: unknown): value is CheckBbox {
   if (typeof value !== "object" || value === null) return false;
   return ["x", "y", "width", "height"].every((key) => typeof Reflect.get(value, key) === "number");