feat(cli): declarative motion verification in inspect (#1437) (#1459)

Extend `inspect` to verify motion intent against the same seeked timeline
the renderer uses, catching render-≠-preview bugs that layout sampling can't:
entrance reveals the seek skips, broken stagger order, off-frame drift, and
frozen shots.

A `*.motion.json` sidecar next to the composition opts in (auto-discovered,
no flag, no authoring-framework changes); without one, inspect is unchanged.
inspect seeks a dense grid over the asserted selectors, builds an
element × time matrix of {rect, opacity, visible} plus per-scope liveness
signatures, and evaluates four assertions in Node:

  appearsBy    -> motion_appears_late
  before       -> motion_out_of_order
  staysInFrame -> motion_off_frame
  keepsMoving  -> motion_frozen

A selector matching nothing is reported as motion_selector_missing rather
than silently passing. Findings reuse the LayoutIssue shape and flow through
the existing dedupe/collapse/limit/format pipeline and JSON envelope; they
are errors by default, so a failed assertion fails the run.

The motion pass runs in the same Chrome session as the layout audit (no extra
launch) and only when a sidecar is present.

.fallowrc.jsonc

@@ -25,6 +25,7 @@
     // path, never imported, so they have no import-graph referrer.
     "packages/cli/src/commands/layout-audit.browser.js",
     "packages/cli/src/commands/contrast-audit.browser.js",
+    "packages/cli/src/commands/motion-sample.browser.js",
     // Worker entry points loaded dynamically by their *Pool.ts companions.
     "packages/producer/src/services/pngDecodeBlitWorker.ts",
     "packages/producer/src/services/shaderTransitionWorker.ts",