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>

README.md

@@ -13,7 +13,7 @@ order.
 | 1 — Prove the ADB link | ✅ proven — see [`docs/phase-1-adb-link.md`](docs/phase-1-adb-link.md) |
 | 2 — Act primitives | ✅ done — tap by text, type, scroll, back, home, launch |
 | 3 — Spotify recipe | ✅ done — `spotify_play` plays the right song end to end (13 steps, 35s), see [`docs/phase-3-spotify-recipe.md`](docs/phase-3-spotify-recipe.md) |
-| 4 — Song radio | not started |
+| 4 — Song radio | ✅ done — `spotify_radio` starts a real radio (19 steps, 57s); the run log is now bounded at ~1 MB × 5, see [`docs/phase-4-song-radio.md`](docs/phase-4-song-radio.md) |
 | 5 — Maps recipe | not started |
 | 6 — Action API | not started |
 | 7 — Resilience | not started |
@@ -71,18 +71,23 @@ what to find on screen; there are no coordinates anywhere.
 ```sh
 ./bin/artemis recipes                                     # list them
 ./bin/artemis run spotify_play query="Rolling Stones Honky Tonk"
+./bin/artemis run spotify_radio query="Rolling Stones Honky Tonk"
 ```
 
 Read [`docs/phase-3-spotify-recipe.md`](docs/phase-3-spotify-recipe.md) for the
-file format, the verbs, and why list rows need their own step.
+file format, the verbs, and why list rows need their own step, and
+[`docs/phase-4-song-radio.md`](docs/phase-4-song-radio.md) for the context menu,
+`menu_row`, and the two ways a radio run fails on purpose.
 
 ## Rules that the code enforces
 
 - **Find by text, never fixed coordinates.** `tap` resolves a text match to
   bounds and taps the centre; `tap_row` scores result rows against the query
-  and taps the winning row's centre.
+  and taps the winning row's centre; `menu_row` opens the action menu of that
+  same row, from the button inside it.
 - **Never tap blindly.** A missing or disabled element is a loud failure, not a
-  guessed coordinate. A named bad screen (`guard`) fails with its own reason.
+  guessed coordinate. A named bad screen (`guard`) fails with its own reason,
+  and a step may name its own failure with `else "..."`.
 - **Never touch a locked phone.** A secure keyguard stops Artemis with
   `phone is locked; unlock it and retry`. No PIN is stored, read or typed
   (AGENTS.md section 13 Q2).
@@ -94,3 +99,6 @@ file format, the verbs, and why list rows need their own step.
 - **Bound every command.** No adb call runs without a timeout, and a recipe has
   a per-step and a whole-run timeout (FR-11).
 - **Log every run.** Each run appends a JSON line to `logs/runs.jsonl` (FR-10).
+  The file rotates at about 1 MB, keeping the recent runs (FR-10 is an audit
+  trail, so history is bounded, never deleted wholesale). Tune it with
+  `ARTEMIS_LOG_MAX_BYTES` and `ARTEMIS_LOG_KEEP`.