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/radars/adaptive_melee_radar.nim

@@ -49,14 +49,6 @@ const
   FreshStreakTicks* = 3       ## consecutive fresh ticks required to enter
                               ## tracking (hysteresis against a single lucky
                               ## fresh tick).
-  CorpseTicks* = 40           ## ticks; an enemy unseen for longer is treated as
-                              ## dead and dropped from the arc/coverage. The
-                              ## staleness fallback re-acquires a LIVE enemy
-                              ## within FreshnessTicks + one full spin, so a
-                              ## live enemy is never unseen this long. Needed
-                              ## because BotDeathEvent does not reach this bot
-                              ## in the current API, so the tracker keeps dead
-                              ## enemies 'alive' forever.
   MarginDeg* = 20.0           ## deg added to EACH end of the covering arc. An
                               ## enemy at 8 px/tick and 300 px changes bearing
                               ## by at most ~1.5 deg/tick; over a half-sweep
@@ -199,9 +191,12 @@ proc computeScan*(m: var AdaptiveMeleeRadarModule, state: WorldState): float =
     if e.id notin m.knownIds:
       m.knownIds.incl e.id
       newId = true
+    # No corpse filter: `state.enemies` is built from `EnemyTracker.allAlive()`
+    # and `onBotDeath` reliably marks deaths (measured 0 corpse ticks; see
+    # docs/tracker_death_events.md). A long-unseen entry is therefore a stale
+    # LIVE enemy, and the freshness fallback below must re-acquire it rather
+    # than the radar silently dropping it from the arc.
     let age = state.tick - e.lastSeenTick
-    if age > CorpseTicks:
-      continue  # dead-but-unmarked corpse: ignore it entirely
     bearings.add normalizeDeg(
       arctan2(e.y - state.selfY, e.x - state.selfX).radToDeg)
     if age > FreshnessTicks: