gun selector: hysteresis measured NEGATIVE, shipped at the lightest setting

Hypothesis under test: the selector chatters (~54 switches/100 ticks) and that
chatter suppresses firing, so committing to the virtual-best gun should raise
real hit rate. MEASURED AGAINST THE REAL DRUSSGT: it does not.

  setting              switches/100t   real hit %   dmg/run   shots/run
  no hysteresis 0/0         54.29        7.02%        217       244.8
  light 10/0.05              1.95        6.22%        191       235.3
  moderate 30/0.15           1.33        5.10%        156       233.9
  aggressive 60/0.30           -         5.72%        175       242.5
(16 runs x 7 rounds per config except aggressive = 8; server-side events
sidecar; permutation test baseline-vs-moderate p=0.002, baseline-vs-light
p=0.18.)

Hysteresis cuts chatter 28-54x but every variant fires slightly FEWER shots and
deals LESS damage than baseline. Mechanism [INFERRED, consistent with
docs/gun_rack_analysis.md 2/4]: the per-tick random tie-break among the tied
band is a hedge, and hysteresis destroys it by committing to the virtual-best
gun - which is not the real-best, because the virtual metric is a weak,
sign-unstable ranker. The chattering was load-bearing.

Shipped: GunDwellTicks=10, GunSwitchMargin=0.05 (GUN_SELECTOR_DWELL /
GUN_SELECTOR_MARGIN) - the only setting within the baseline's run-to-run spread.
GUN_SELECTOR_DWELL=0 GUN_SELECTOR_MARGIN=0 reproduces the pre-change selector
exactly.

Seam: VirtualTracker, which already owns the other selection state (fitness,
the relative floor's peakRateRef), so the bot needs no new fields. bestGun and
chooseFromFit stay pure/memoryless, which is why the existing random-tiebreak
test needed no change.

Guards: test_gun_harness 39/39 (33 original + 6 new hysteresis checks),
test_vbullet_metric, test_power_selection, acceptance_offline_vs_online 12/12.

common_libs/tests/test_gun_harness.nim

@@ -84,6 +84,59 @@ proc testBestGunDeterministicWinner() =
   check "gun with clearly best rate and >= MinObsBeforeCompete wins deterministically",
         allTwo
 
+# ── sticky selector (hysteresis) ──────────────────────────────────────────────
+# `selectGun` is the live path; `bestGun`/`chooseFromFit` stay memoryless. These
+# checks pin the dwell, switch-margin, disqualification and floor behaviours.
+
+proc testHysteresisDwell() =
+  var t = initTracker(2)
+  seedWindow(t, 7, gunId = 0, binIdx = 0, hits = 60,  misses = 40)  # 60%
+  seedWindow(t, 7, gunId = 1, binIdx = 0, hits = 100, misses = 0)   # 100%
+  t.currentGun = 0
+  t.currentSince = 0
+  check "hysteresis: a clearly better gun is held off inside the dwell window",
+        t.selectGun(7, tick = 5) == 0
+  check "hysteresis: dwell expiry lets the better gun take over",
+        t.selectGun(7, tick = GunDwellTicks) == 1
+
+proc testHysteresisSwitchMargin() =
+  # 80% incumbent vs 83% challenger: inside a 5% relative margin -> hold.
+  var t = initTracker(2)
+  seedWindow(t, 7, gunId = 0, binIdx = 0, hits = 80, misses = 20)
+  seedWindow(t, 7, gunId = 1, binIdx = 0, hits = 83, misses = 17)
+  t.currentGun = 0
+  t.currentSince = 0
+  check "hysteresis: a challenger inside the switch margin does not displace the incumbent",
+        t.selectGun(7, tick = 100) == 0
+
+  # 80% incumbent vs 90% challenger: beyond the margin -> switch.
+  var t2 = initTracker(2)
+  seedWindow(t2, 7, gunId = 0, binIdx = 0, hits = 80, misses = 20)
+  seedWindow(t2, 7, gunId = 1, binIdx = 0, hits = 90, misses = 10)
+  t2.currentGun = 0
+  t2.currentSince = 0
+  check "hysteresis: a challenger beyond the switch margin displaces the incumbent",
+        t2.selectGun(7, tick = 100) == 1
+
+proc testHysteresisDisqualified() =
+  var t = initTracker(2)
+  seedWindow(t, 7, gunId = 0, binIdx = 0, hits = 10, misses = 0)  # < MinObsBeforeCompete
+  seedWindow(t, 7, gunId = 1, binIdx = 0, hits = 60, misses = 0)
+  t.currentGun = 0
+  t.currentSince = 0
+  check "hysteresis: a disqualified incumbent is replaced even inside dwell",
+        t.selectGun(7, tick = 3) == 1
+
+proc testHysteresisFloor() =
+  var t = initTracker(2)
+  seedWindow(t, 7, gunId = 0, binIdx = 0, hits = 0, misses = 60)
+  seedWindow(t, 7, gunId = 1, binIdx = 0, hits = 0, misses = 60)
+  t.currentGun = 1
+  t.currentSince = 0
+  t.peakRateRef = 1.0
+  check "hysteresis: floor collapse returns HeadOn even mid-dwell",
+        t.selectGun(7, tick = 3) == 0
+
 proc testBestPowerCold() =
   var t = initTracker(3)
   let (bin, power) = t.bestPower(0, -1)
@@ -357,6 +410,10 @@ randomize()
 testColdBestGun()
 testRandomTiebreak()
 testBestGunDeterministicWinner()
+testHysteresisDwell()
+testHysteresisSwitchMargin()
+testHysteresisDisqualified()
+testHysteresisFloor()
 testBestPowerCold()
 testBestPowerWarmBin3()
 testFitnessForDeterministic()