Trace a face over its footage, and choose where its origin goes

A face's :head carries :trace {:frames :origin}: the frames its photo holds
on, and whether the head reads every frame, jumps to the trace frames, or
holds frame 0. It replaces :anchors, so which measured frame a head reads is
one stored fact. An instance's :underlay shows the tracing stills over every
face at or below it, registered through each face's own head, at an opacity,
unkeyed. The clip resolver answers where a row path went on its last frame,
so the paint loop reads the photo's matrix instead of resolving again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/test/arthur/domain/gesture_test.cljs

@@ -90,7 +90,6 @@
 
 (deftest a-measured-transform-is-not-set-by-hand
   (is (string? (gesture/refusal {:channels {[:xform :pos] {:animated? true :dense {:stride 2}}}})))
-  (is (string? (gesture/refusal {:anchors {0 12}})))
   (is (nil? (gesture/refusal {:channels {[:xform :pos] (ch/keyed {0 [1 1]})}}))))
 
 (deftest a-click-selects-the-level-figma-would