tune(SNNBot): bullet economy tuning vs Walls — 2/10 wins

Changes:
- Fixed fire power at 3.0 (removes hitRateEMA power ladder noise that
  cleared exemplars mid-battle and corrupted learning)
- Relative velocity encoding: input uses (velDir - bearing) instead of
  absolute velDir, so exemplars generalize across Walls' starting walls
- Fix test winner detection to use per-round score delta instead of
  rank field (rank in round_ended is cumulative battle rank, not round winner)
- Keep exemplars across power changes (no longer relevant with fixed power)
- Store lastBulletSpeed at fire time for accurate EVALUATE lead prediction

Result: 2/10 rounds won vs Walls (Nim); first-round win now possible from
round 1 when aimer generalizes from relative velocity patterns.
Bottleneck: sparse exemplars in early rounds; energy bleeds at P3 cold-start.

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

SNNBot_garage/tests/test_bullet_economy.nim

@@ -36,15 +36,20 @@ let r = runBattle(@[snnbotDir, wallsbotDir], rounds = 10)
 # Per-round stats
 echo "=== Per-round results ==="
 var snnWins, wallsWins: int
+var prevSnn, prevWalls: int
 for rnd in r.rounds:
   var snn, walls: BotRoundResult
   for br in rnd.results:
     if br.name == "SNNBot":      snn   = br
     if br.name == "Walls (Nim)": walls = br
-  # rank 1 = winner; lower rank is better; default rank=0 means bot not found
-  let winner = if snn.rank > 0 and (walls.rank == 0 or snn.rank < walls.rank): "SNNBot" else: "Walls"
+  let snnDelta   = snn.score   - prevSnn
+  let wallsDelta = walls.score - prevWalls
+  prevSnn   = snn.score
+  prevWalls = walls.score
+  # Use per-round score delta to determine round winner (rank field is cumulative battle rank)
+  let winner = if snnDelta > wallsDelta: "SNNBot" else: "Walls"
   if winner == "SNNBot": inc snnWins else: inc wallsWins
-  echo fmt"Round {rnd.round:2}: {winner} wins | SNNBot score={snn.score} survived={snn.survived} | Walls score={walls.score} survived={walls.survived}"
+  echo fmt"Round {rnd.round:2}: {winner} wins | SNNBot +{snnDelta}(={snn.score}) rank={snn.rank} survived={snn.survived} | Walls +{wallsDelta}(={walls.score}) rank={walls.rank} survived={walls.survived}"
 
 # Summary
 echo ""