feat(ModularBot): melee targeting — multi-enemy tracker, per-enemy gun fitness, radar auto-switch

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

common_libs/targeting/target_selector.nim

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+## Target selector: picks which enemy to aim at.
+
+import enemy_tracker
+
+type TargetMode* = enum
+  tmClosest,   ## nearest enemy
+  tmWeakest,   ## lowest energy (finisher)
+  tmBestGun    ## future — alias to closest for now
+
+proc selectTarget*(et: EnemyTracker, selfX, selfY: float,
+                   mode: TargetMode = tmClosest): int =
+  ## Returns enemy bot ID. Falls back to 1v1 naturally when 1 alive.
+  let alive = et.allAlive()
+  if alive.len == 0: return -1
+  if alive.len == 1: return alive[0].id
+  let chosen = case mode
+    of tmWeakest: et.weakest()
+    else:         et.closestTo(selfX, selfY)  # tmClosest + tmBestGun
+  result = chosen.id