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/targeting/enemy_tracker.nim

@@ -1,8 +1,7 @@
 ## Per-enemy state table. Keyed by bot ID.
 ## Tank Royale: 0° = East.
 
-import std/tables
-import std/math
+import std/[tables, math]
 
 type
   EnemyState* = object
@@ -28,6 +27,13 @@ proc markDead*(et: var EnemyTracker, botId: int) =
   if botId in et.enemies:
     et.enemies[botId].alive = false
 
+proc isAlive*(et: EnemyTracker, botId: int): bool =
+  botId in et.enemies and et.enemies[botId].alive
+
+proc aliveCount*(et: EnemyTracker): int =
+  for s in et.enemies.values:
+    if s.alive: inc result
+
 proc resetRound*(et: var EnemyTracker) =
   et.enemies.clear()
 
@@ -36,7 +42,7 @@ proc getEnemy*(et: EnemyTracker, botId: int): EnemyState =
 
 proc allAlive*(et: EnemyTracker): seq[EnemyState] =
   for s in et.enemies.values:
-    if s.alive: result.add(s)
+    if s.alive: result.add s
 
 proc closestTo*(et: EnemyTracker, x, y: float): EnemyState =
   var bestDist = Inf