feat: Phase 4 — spotify_radio, and a bound on logs/runs.jsonl

spotify_radio opens a track's own context menu and starts its song radio.
The labels came from a fresh uiautomator dump on the phone, not from a guess:
the entry is "Go to song radio" and it sits below the fold, so the recipe
scrolls before it matches.

Recipe format:
- menu_row "{query}" finds the row like tap_row, then presses the action
  button inside that row's own subtree (ui.Row.Action), so the More options
  button of any other row is never a candidate. It records the row as {row}.
- a step can name its own failure: wait ... else "reason". The named reason
  comes first and the screen's labels still follow (FR-8).
- scroll up|down moves the current list.
- playing with no title waits for the media session to be PLAYING. A radio
  changes track every few minutes, so a title match would fail on a working
  radio; spotify_play still passes the title it wants.

ui:
- pickBestMatch ignores a match with no usable bounds. An element scrolled out
  of its list reports [0,0][0,0], and tapping its "centre" would be a blind tap
  in the corner. A wait now only succeeds once the element is on screen.

Run log (FR-10): appendRunLog rotates runs.jsonl when a write would pass
ARTEMIS_LOG_MAX_BYTES (default 1 MiB), keeping ARTEMIS_LOG_KEEP (default 5)
files. History is bounded, never deleted wholesale, and the per-entry cap of
6 screen labels of 60 characters is unchanged.

Live evidence in docs/phase-4-song-radio.md: ok:true in 56.6s, the menu that
opened belonged to the pressed row, the station name "Honky Tonk Women Radio"
on the phone, and the radio playing past its seed track (Layla) per the media
session and dumpsys audio. Both new failure modes were demonstrated failing
with distinct reasons. The mini-bar's transport label stays "Play" while
playing on this build, which is why playback is confirmed from the media
session.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/ui/actions.go

@@ -92,15 +92,25 @@ type tapCandidate struct {
 // pickBestMatch chooses the most tappable node from a set of text matches.
 // Clickable and enabled nodes win; among equals the smallest node wins, because
 // the smallest enclosing element is the control itself rather than its container.
+//
+// A node without usable bounds is never a tap target. An element that is not on
+// screen (scrolled out of a list, or not laid out yet) reports bounds of
+// [0,0][0,0], and tapping its "centre" would be a blind tap at the top-left of
+// the screen. Such a match is skipped, and a set with no usable match fails
+// (FR-8) rather than tapping anything.
 func pickBestMatch(matches []*Node) (*Node, error) {
 	candidates := make([]tapCandidate, 0, len(matches))
 	for _, n := range matches {
-		area := 1 << 30
-		if r, err := n.BoundsRect(); err == nil {
-			area = (r.X2 - r.X1) * (r.Y2 - r.Y1)
+		r, err := n.BoundsRect()
+		if err != nil {
+			continue
 		}
+		area := (r.X2 - r.X1) * (r.Y2 - r.Y1)
 		candidates = append(candidates, tapCandidate{node: n, clickable: n.Clickable, enabled: n.Enabled, area: area})
 	}
+	if len(candidates) == 0 {
+		return nil, fmt.Errorf("the matched element has no usable bounds: it is not on screen; refusing to tap blindly")
+	}
 	sort.SliceStable(candidates, func(i, j int) bool {
 		a, b := candidates[i], candidates[j]
 		if a.clickable != b.clickable {
@@ -168,13 +178,13 @@ func WaitForText(r *adb.Runner, query string, exact bool, timeout time.Duration)
 		timeout = 10 * time.Second
 	}
 	deadline := time.Now().Add(timeout)
-	var seen []*Node
+	sawMatch := false
 	for time.Now().Before(deadline) {
 		h, err := Dump(r)
 		if err == nil {
 			matches := h.FindText(query, exact)
 			if len(matches) > 0 {
-				seen = matches
+				sawMatch = true
 				if n, perr := pickBestMatch(matches); perr == nil {
 					return n, nil
 				}
@@ -182,8 +192,8 @@ func WaitForText(r *adb.Runner, query string, exact bool, timeout time.Duration)
 		}
 		time.Sleep(PollInterval)
 	}
-	if len(seen) > 0 {
-		return nil, fmt.Errorf("text %q is on screen but no enabled match could be tapped", query)
+	if sawMatch {
+		return nil, fmt.Errorf("text %q is present but never became a usable tap target within %s (it is off screen or disabled)", query, timeout)
 	}
 	return nil, fmt.Errorf("text %q did not appear within %s", query, timeout)
 }