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/rows_test.go

@@ -215,3 +215,54 @@ func TestTokenize(t *testing.T) {
 		}
 	}
 }
+
+// TestRowActionFindsTheButtonsInsideTheChosenRow is the safety property behind
+// menu_row: the More options button is taken from the row's own subtree, so a
+// recipe can never press the button of a different row.
+func TestRowActionFindsTheButtonsInsideTheChosenRow(t *testing.T) {
+	h := spotifyRows(t)
+	row, err := h.BestRow("Rolling Stones Honky Tonk", spotifyRowID, 1080, 2400)
+	if err != nil {
+		t.Fatalf("BestRow: %v", err)
+	}
+	if row.Title != "Honky Tonk Women" {
+		t.Fatalf("chose %q", row.Title)
+	}
+
+	button, err := row.Action("More options")
+	if err != nil {
+		t.Fatalf("Action: %v", err)
+	}
+	// The top hit's menu button, not the duplicate row's at y=1380.
+	if button.Bounds != "[828,815][954,941]" {
+		t.Fatalf("chose button %s, want the top row's", button.Bounds)
+	}
+	if !strings.Contains(button.ContentDesc, "More options for song Honky Tonk Women") {
+		t.Fatalf("chose %q", button.ContentDesc)
+	}
+
+	// A label the row does not carry fails instead of falling back to any
+	// clickable node.
+	if _, err := row.Action("Share"); err == nil {
+		t.Fatal("expected an error for a label the row does not have")
+	}
+}
+
+// TestRowActionRefusesARowWithoutAButton covers a result row that has no action
+// bar at all: the step must fail, not tap the row itself.
+func TestRowActionRefusesARowWithoutAButton(t *testing.T) {
+	h := spotifyRows(t)
+	var bare *Node
+	h.Walk(func(n *Node) {
+		if n.ResourceID == spotifyRowID && n.Bounds == "[0,962][1080,1130]" {
+			bare = n
+		}
+	})
+	if bare == nil {
+		t.Fatal("the sample row without an action bar was not found")
+	}
+	row := &Row{Node: bare, Title: "Honky Tonk Women - Mono Version"}
+	if _, err := row.Action("More options"); err == nil {
+		t.Fatal("a row with no action bar must not yield a button")
+	}
+}