feat(gun_harness): per-gun REAL hit attribution + bestPower cold-start fix

Attribution is proven, not guessed: the server assigns a per-round-unique
bulletId (GunEngine.nextBulletId) and stamps the same id on BulletFired,
BulletHitBot, BulletHitWall and BulletHitBullet. Keep a FIFO of fired gun
ids, stamp bulletId -> gunId on onBulletFired, resolve through that map.

Hits are deferred when onBulletHit precedes onBulletFired in the same
turn (client dispatches priority 70 > 60), which recovered 14
unattributed hits. 99.9% of shots and 99.8% of hits attributed.

Stats lines now carry per-gun realShots/realHits/realHitRate; the old
keys and round-level totals are unchanged.

bestPower: a gun with zero observations in every bin previously returned
the HIGHEST bin (power 3.0) because an empty bin satisfied the
'count == 0' clause on the first countdown iteration. Cold guns now
return the lowest bin as the docstring always claimed. Warm-gun path
untouched.

common_libs/gun_harness/virtual_bullets.nim

@@ -161,6 +161,18 @@ proc bestPower*(t: VirtualTracker, gunId: GunId, targetId: int = -1): (int, floa
   ## Uses per-enemy fitness when targetId >= 0 and data exists; else aggregate.
   let fit = t.fitnessFor(targetId)
   result = (0, PowerBins[0])
+  # Cold gun (zero observations in every bin): fall back to the lowest power bin,
+  # as documented. Without this the countdown loop below would hit the empty
+  # highest bin first and wrongly return power 3.0.
+  var anyObs = false
+  for binIdx in 0..<len(PowerBins):
+    if fit[gunId].bins[binIdx].count > 0:
+      anyObs = true
+      break
+  if not anyObs:
+    return (0, PowerBins[0])
+  # Warm gun: unchanged — return the highest power bin clearing MinHitRate
+  # (or an empty bin, which the existing logic treats as acceptable).
   for binIdx in countdown(len(PowerBins) - 1, 0):
     let rate = fit[gunId].bins[binIdx].hitRate()
     if rate >= MinHitRate or fit[gunId].bins[binIdx].count == 0: