fix(PPO_Bot): radar lock oscillation bug — arctan2 arg order wrong for Tank Royale coords

arctan2(dy, dx) gives east-based math bearing; Tank Royale uses north=0°, CW+.
Swapping to arctan2(dx, dy) gives the correct game-space bearing.
Symptom: radar commanded 45°/tick away from a target directly ahead.
Adds test_radar_lock.nim as regression test (20-tick lock, ±15° tolerance).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

PPO_Bot/enemy_tracker.nim

@@ -82,9 +82,10 @@ proc getRadarTurnRate*(tracker: var EnemyTracker;
     return 45.0
 
   # Bearing from radar to enemy
+  # Tank Royale: 0°=north, CW+. arctan2(dx, dy) gives north-based CW bearing.
   let dx = tracker.current.x - botX
   let dy = tracker.current.y - botY
-  let absoluteDir = (180.0 * arctan2(dy, dx) / PI + 360.0) mod 360.0
+  let absoluteDir = (180.0 * arctan2(dx, dy) / PI + 360.0) mod 360.0
   let radarBearing = normalizeRelative(absoluteDir - radarDirection)
 
   # Overshoot by 10°, alternate direction each tick

PPO_Bot/tests/test_radar_lock.nim

@@ -0,0 +1,42 @@
+## Regression test: radar lock must hold on a stationary target.
+## Run: nim c -r tests/test_radar_lock.nim
+
+import std/[math, strformat]
+import "../enemy_tracker"
+
+template check(cond: bool, msg: string) =
+  if not cond:
+    quit("FAIL: " & msg, 1)
+
+# Bot at arena center; stationary enemy due north (same x, higher y).
+# Tank Royale: y increases northward.
+# True bearing to enemy = 0° (north = 0° in game coords).
+
+let botX = 400.0
+let botY = 300.0
+let enemyX = 400.0   # same x → dx = 0
+let enemyY = 500.0   # north of bot → dy > 0
+let trueBearing = 0.0  # north
+
+# Radar starts pointing at the enemy (radarDirection = 0°, due north).
+var radarDir = 0.0
+
+var tracker = initEnemyTracker()
+# Prime with contact at the known position
+tracker.update(enemyX, enemyY, 0.0, 0.0, 100.0)
+
+echo "Tick | radarDir | trueBearing | bearingErr"
+for tick in 1 .. 20:
+  let rate = tracker.getRadarTurnRate(botX, botY, 0.0, radarDir)
+  radarDir = (radarDir + rate + 360.0) mod 360.0
+
+  # Simulate a successful scan every tick (enemy is stationary)
+  tracker.update(enemyX, enemyY, 0.0, 0.0, 100.0)
+
+  # Bearing error: signed difference, wrapped to [-180, 180]
+  let err = ((radarDir - trueBearing) + 540.0) mod 360.0 - 180.0
+  echo &"  {tick:2d} | {radarDir:8.3f}° | {trueBearing:8.3f}° | {err:+.3f}°"
+
+  check abs(err) <= 15.0, &"tick {tick}: radar {radarDir:.1f}° drifted > 15° from target {trueBearing:.1f}°"
+
+echo "All radar lock tests passed"