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/commands/check.ts

@@ -147,15 +147,12 @@ export function createCheckCommand(
 }
 
 function parseCheckOptions(args: Record<string, unknown>): CheckOptions {
-  const maxTransitionSamplesRaw = parseInt(String(args["max-transition-samples"] ?? ""), 10);
+  const maxTransitionSamples = positiveInteger(args["max-transition-samples"], 0);
   return {
     samples: positiveInteger(args.samples, DEFAULT_CHECK_OPTIONS.samples),
     at: parseAt(args.at),
     atTransitions: args["at-transitions"] === true,
-    maxTransitionSamples:
-      Number.isFinite(maxTransitionSamplesRaw) && maxTransitionSamplesRaw > 0
-        ? maxTransitionSamplesRaw
-        : undefined,
+    maxTransitionSamples: maxTransitionSamples > 0 ? maxTransitionSamples : undefined,
     maxIssues: positiveInteger(args["max-issues"], DEFAULT_CHECK_OPTIONS.maxIssues),
     collapseStatic: args["collapse-static"] !== false,
     tolerance: nonNegativeNumber(args.tolerance, DEFAULT_CHECK_OPTIONS.tolerance),