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/measure_hysteresis.nim

@@ -0,0 +1,73 @@
+## Offline chatter measurement for the gun selector's hysteresis.
+##
+## Replays real recorded DrussGT movement (and the synthetic set) through the 13
+## ModularBot guns and drives the SAME stateful selector the live bot uses
+## (`selectGun`), counting how often the chosen gun changes. This is the
+## deterministic analogue of counting the live `[config]` gun-switch lines: the
+## replay feeds the identical per-tick fitness the live tracker builds, so the
+## switch sequence is the same up to the seeded tie-break RNG.
+##
+## The env knobs (`GUN_SELECTOR_DWELL`, `GUN_SELECTOR_MARGIN`) are read once per
+## process at module init, so sweep by running the binary under different values:
+##
+##   nim c -r common_libs/tests/measure_hysteresis.nim                       # shipped
+##   GUN_SELECTOR_DWELL=0 GUN_SELECTOR_MARGIN=0 \
+##       common_libs/tests/measure_hysteresis                   # pre-hysteresis baseline
+##
+## This is a MEASUREMENT tool, not a test; it always exits 0.
+
+import std/[os, strformat, algorithm, random]
+import gun_harness/offline_range
+import gun_harness/virtual_bullets
+import range_guns
+
+const
+  repoRoot = currentSourcePath().parentDir.parentDir.parentDir
+  fixturesDir = repoRoot / "tools" / "fixtures"
+
+type RunStat = object
+  name:     string
+  ticks:    int
+  switches: int
+
+proc measure(path: string, seed: int): RunStat =
+  let fx = loadFixture(path)
+  let drivers = buildAllGunDrivers(seed = seed, enableTmSelector = false)
+  var last = -1
+  var t = 0
+  var switches = 0
+  discard replayFixture(fx, drivers,
+    liveActual = (fx.meta.source == "live"),
+    tickCb = proc(tr: ptr VirtualTracker) =
+      let g = selectGun(tr[], fx.enemyId, t)
+      if last >= 0 and g != last: inc switches
+      last = g
+      inc t)
+  RunStat(name: extractFilename(path), ticks: fx.states.len, switches: switches)
+
+proc main() =
+  randomize(12345)
+  var paths: seq[string]
+  for f in walkFiles(fixturesDir / "drussgt_vs_*.jsonl"): paths.add f
+  for f in walkFiles(fixturesDir / "tr_drussgt_vs_*.jsonl"): paths.add f
+  paths.sort()
+
+  var totalTicks = 0
+  var totalSwitches = 0
+  echo fmt"dwell={ActiveDwellTicks} ticks  margin={ActiveSwitchMargin:.3f}  " &
+       fmt"mode={ActiveSelectorMode}  rank={ActiveRank}"
+  echo "fixture                              ticks  switches  sw/100t"
+  echo "----------------------------------------------------------------"
+  var stats: seq[RunStat]
+  for p in paths:
+    let s = measure(p, seed = 12345)
+    stats.add s
+    totalTicks += s.ticks
+    totalSwitches += s.switches
+    echo fmt"{s.name:<34}{s.ticks:>7}{s.switches:>10}{100.0*float(s.switches)/float(max(1,s.ticks)):>9.2f}"
+  echo "----------------------------------------------------------------"
+  let label = "TOTAL"
+  echo fmt"{label:<34}{totalTicks:>7}{totalSwitches:>10}{100.0*float(totalSwitches)/float(max(1,totalTicks)):>9.2f}"
+
+when isMainModule:
+  main()