feat(PPO_Bot): weight persistence + background training (#17)

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

PPO_Bot/PPO_Bot.nim

@@ -1,16 +1,18 @@
 ## PPO_Bot — enemy tracker + state vector wired into the game loop.
-## Training: trajectory collected per tick, PPO update on round end.
+## Training: trajectory collected per tick, PPO update in background thread.
 
-import std/os
+import std/[os, locks]
 import arraymancer
 import tankroyale_botapi
 import network
 import actions
 import training
+import weights
 import ./enemy_tracker
 import ./state_vector
 
 const botJsonPath = currentSourcePath().parentDir / "PPO_Bot.json"
+const weightsRoot = currentSourcePath().parentDir / "weights"
 
 type PPOBot = ref object of Bot
   tracker:    EnemyTracker
@@ -25,6 +27,32 @@ type PPOBot = ref object of Bot
 
 var ac = initActorCritic()
 
+# ── Background training state ─────────────────────────────────────────────────
+
+type TrainingArgs = object
+  ac:          ActorCritic
+  buffer:      TrajectoryBuffer
+  lastValue:   float32
+  roundNum:    int
+  weightsRoot: string
+
+var
+  trainingThread: Thread[TrainingArgs]
+  trainingDone:   bool = true    # true = no training running / last run finished
+  trainingLock:   Lock
+  resultChan:     Channel[ActorCritic]
+  roundCounter:   int = 0
+
+proc trainingThreadProc(args: TrainingArgs) {.thread.} =
+  var localAc = args.ac
+  ppoUpdate(localAc, args.buffer, lastValue = args.lastValue)
+  saveCheckpoint(localAc, args.weightsRoot, args.roundNum)
+  resultChan.send(localAc)
+  withLock(trainingLock):
+    trainingDone = true
+
+# ── Bot methods ───────────────────────────────────────────────────────────────
+
 method onScannedBot*(bot: PPOBot, e: ScannedBotEvent) =
   bot.tracker.update(e.x, e.y, e.direction, e.speed, e.energy)
 
@@ -36,17 +64,45 @@ method onRoundStarted*(bot: PPOBot, e: RoundStartedEvent) =
   bot.hasLastTrans  = false
 
 method onRoundEnded*(bot: PPOBot, e: RoundEndedEventForBot) =
+  inc roundCounter
+
   # Add round-end score bonus to last transition (if any)
   let roundReward = computeRoundReward(e.results.totalScore.float32)
   if bot.hasLastTrans and bot.buffer.len > 0:
     bot.buffer.transitions[^1].reward += roundReward
 
-  # PPO update synchronous; background thread is issue #17
-  # ponytail: blocking update per round; move to thread pool when #17 lands
-  ppoUpdate(ac, bot.buffer, lastValue = 0.0'f32)
+  # Pick up result from previous training thread if done
+  withLock(trainingLock):
+    if trainingDone and roundCounter > 1:
+      let (avail, trained) = resultChan.tryRecv()
+      if avail:
+        ac = trained
+
+  if bot.buffer.len == 0:
+    bot.hasLastTrans = false
+    return
+
+  # If last thread still running, drop this pass — start fresh with newer data
+  # ponytail: simple drop; queue if every round must train
+  withLock(trainingLock):
+    if not trainingDone:
+      bot.buffer.clear()
+      bot.hasLastTrans = false
+      return
+    trainingDone = false
+
+  let args = TrainingArgs(
+    ac:          ac,
+    buffer:      bot.buffer,
+    lastValue:   0.0'f32,
+    roundNum:    roundCounter,
+    weightsRoot: weightsRoot,
+  )
   bot.buffer.clear()
   bot.hasLastTrans = false
 
+  createThread(trainingThread, trainingThreadProc, args)
+
 method run(bot: PPOBot) =
   # Seed energy on first tick
   bot.prevEnergy = getEnergy().float32
@@ -110,6 +166,12 @@ method run(bot: PPOBot) =
     go()
 
 when isMainModule:
+  initLock(trainingLock)
+  resultChan.open()
+  createDir(weightsRoot)
+  cleanStaleTempDirs(weightsRoot)
+  discard loadBestAvailable(ac, weightsRoot)
+
   var bot = PPOBot(
     tracker: initEnemyTracker(),
     buffer:  initTrajectoryBuffer(),