tracker: corpses do not exist - revert the fix and retire the workaround

The belief "BotDeathEvent never reaches ModularBot, so enemyTracker keeps dead
enemies alive forever" was written into a code comment and then believed twice.
It is FALSE. Measured in a 7-bot melee with a per-tick probe comparing
enemyTracker's alive count against the server's getEnemyCount():

  metric                          1.3.1 (20 rd)   0.35.5 (15 rd)
  observed enemy deaths                83              68
  ...non-round-ending              83 (100%)       66 (97%)
  ekBotDeath events DROPPED             0               0
  max dispatch lag (turns behind)       1               1
  phantom ticks                  1 / 16,820      1 / 12,596
  MAX CORPSE LIFETIME               0 ticks         0 ticks
  victims still alive at round end      0               0

onBotDeath fires for every death, including non-round-ending ones. The
API-level event-drop mechanism IS real (test_event_drop_mechanism.nim proves
it: ekBotDeath is not in isCritical and MAX_EVENTS_AGE=2) - the bot simply
never falls far enough behind for it to trigger (max lag 1 turn).

Removed:
- reconcileWithServer + ReconcilePersistTicks/mismatchTicks/sawServerAlive
  (uncommitted, and ON BY DEFAULT despite the premise being false). Its own
  comment admitted a shorter window once KILLED A LIVE ENEMY ("it fired three
  more times after the tracker marked it dead") - a latent mis-prune path
  defending against a bug that does not exist.
- The radar's CorpseTicks=40 filter and the same-class age>60 filter in
  recordRadarStats, both carrying the false comment. Removal changes no real
  behaviour: buildState feeds the radar enemyTracker.allAlive(), so a dead
  enemy never reaches computeScan.

Kept:
- The TR_TRACKER_PROBE instrument (default OFF), which produced the table above.
- test_event_drop_mechanism.nim - the drop mechanism is a genuine library
  behaviour worth guarding.
- isAlive/aliveCount on the tracker.

Added: docs/tracker_death_events.md (the durable negative, so this is not
re-invented a third time) and test_enemy_tracker_death.nim (13 checks) in place
of the test for the deleted feature.

Guards: test_gun_harness 39/39, test_vbullet_metric 11, test_power_selection 3,
test_adaptive_radar 41/41, test_event_drop_mechanism 6, test_enemy_tracker_death
13, acceptance 12/12, ModularBot compiles.

common_libs/tests/test_adaptive_radar.nim

@@ -318,9 +318,11 @@ proc testExpectedDisabled() =
     discard m.computeScan(ws(t, 0.0, @[enemyAt(1, 0.0, 300.0, float(t))]))
   check "gate: expectedEnemies = -1 does not block acquisition", m.phase == rpTrack
 
-proc testCorpseIgnored() =
-  # A dead-but-unmarked enemy (unseen > CorpseTicks) must not keep the radar in
-  # acquisition, nor widen the arc.
+proc testStaleLiveEnemyTrusted() =
+  # With the corpse filter retired the radar trusts the tracker: a tracked
+  # enemy with a very old lastSeenTick is a stale LIVE enemy, not a corpse. It
+  # is still included in the covering arc (so the sweep covers it) and, being
+  # stale, keeps the radar in acquisition until it has been re-scanned.
   var m = initAdaptiveMeleeRadar()
   m.setExpectedEnemies(1)
   var t = 0
@@ -328,27 +330,20 @@ proc testCorpseIgnored() =
     inc t
     discard m.computeScan(ws(t, 0.0, @[
       enemyAt(1, 100.0, 300.0, float(t)),
-      enemyAt(2, 300.0, 300.0, float(t - 100)),  # corpse, far bearing
+      enemyAt(2, 300.0, 300.0, float(t - 100)),  # stale, but alive
     ]))
-  check "corpse: a long-unseen enemy is ignored and tracking is reached",
-        m.phase == rpTrack
-  check "corpse: the corpse does not widen the swept arc",
-        approx(m.lastSweptWidth, 2.0 * MarginDeg)
-
-proc testCorpseExpectedGate() =
-  # expectedEnemies = 2 but only one LIVE enemy (the other is a corpse): the
-  # live count is 1, so the gate keeps the radar in acquisition.
-  var m = initAdaptiveMeleeRadar()
-  m.setExpectedEnemies(2)
-  var t = 0
-  for _ in 0 ..< FreshStreakTicks + 5:
-    inc t
-    discard m.computeScan(ws(t, 0.0, @[
-      enemyAt(1, 100.0, 300.0, float(t)),
-      enemyAt(2, 300.0, 300.0, float(t - 100)),
-    ]))
-  check "corpse: the expected gate counts LIVE enemies only",
+  # Bearings 100 and 300: the minimal covering arc runs 300 -> 100 (width 160).
+  check "trust: a long-unseen tracked enemy is not dropped from the arc",
+        approx(m.lastSweptWidth, 160.0 + 2.0 * MarginDeg)
+  check "trust: a long-unseen tracked enemy keeps the radar in acquisition",
         m.phase == rpAcquire
+  # When the tracker stops reporting it (onBotDeath marked it dead, so it is
+  # absent from `state.enemies`) the radar trusts that and tracks the survivor.
+  for _ in 0 ..< FreshStreakTicks + 2:
+    inc t
+    discard m.computeScan(ws(t, 0.0, @[enemyAt(1, 100.0, 300.0, float(t))]))
+  check "trust: once the dead enemy leaves the tracker the radar tracks",
+        m.phase == rpTrack
 
 # ── driver ───────────────────────────────────────────────────────────────────
 
@@ -373,8 +368,7 @@ testWideArcFallback()
 testWideArcExitAndEnterHysteresis()
 testNoKnownEnemyStaysAcquire()
 testExpectedDisabled()
-testCorpseIgnored()
-testCorpseExpectedGate()
+testStaleLiveEnemyTrusted()
 
 if failures > 0:
   echo "\n", failures, " check(s) FAILED"