transcript-ui: log selection begin/extend, for on-device verification

Selection has no accessibility label of its own yet, so a logcat line at
begin/extend is the smallest way to confirm a real long-press-then-drag
reached DragArbiter/Selection on-device. Driven with the new ui-trace
holddrag action against iris-android-app's transcript screen: produced
"iris selection: begin at row ..." then a sequence of "... extend to row
..." lines, and a screenshot right after shows the expected highlighted
selection spanning multiple rows.

New `log = "0.4.28"` dependency (matching iris-android-app's own pin) --
transcript-ui had no logging facility before this.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

iris/android-app/Cargo.lock

@@ -3867,6 +3867,7 @@ dependencies = [
  "client-core",
  "event-model",
  "iris",
+ "log",
  "pulldown-cmark",
 ]