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/measure_corpse_melee.nim

@@ -0,0 +1,54 @@
+## Corpse measurement: run a real melee where bots kill each other and log
+## ModularBot's tracker-vs-server disagreement per tick.
+##
+## The tracker probe (ModularBot TR_TRACKER_PROBE=1) records, each tick:
+##   tracker_alive (bot.enemyTracker.allAlive().len), server_ec (getEnemyCount()),
+##   each tracked enemy's lastSeenTick, the dead map and any reconcile prunes.
+## The queue probe records the event queue BEFORE removeOldEvents deletes events.
+##
+## TR_TRACKER_RECONCILE defaults to 0 here so the RAW corpse behaviour is
+## measured (a corpse can only be removed by a BotDeathEvent fast-path).
+##
+##   MELEE_ROUNDS=20 TR_SERVER_JAR=<jar> \
+##     nim c -r --path:common_libs common_libs/tests/measure_corpse_melee.nim
+
+import std/[os, strformat, strutils]
+import test_framework/server_manager
+import test_framework/bot_compiler
+import test_framework/runner_process
+import test_framework/battle_result
+
+const
+  repoRoot = currentSourcePath().parentDir.parentDir.parentDir
+  modularBotDir = repoRoot / "ModularBot_garage"
+  samplesDir = "/home/davide/Downloads/robocode-tankroyale/sample-bots-nim-linux-1.0.7"
+
+let drussDir  = getEnv("DRUSSGT_BOTDIR", "/tmp/tr_bots/DrussGT")
+let rounds    = parseInt(getEnv("MELEE_ROUNDS", "20"))
+let probePath = getEnv("TR_TRACKER_PROBE_PATH", "/tmp/tracker_probe_melee.jsonl")
+
+# RAW behaviour: no reconciliation, so any corpse persists until a BotDeathEvent.
+putEnv("TR_TRACKER_RECONCILE", getEnv("TR_TRACKER_RECONCILE", "0"))
+putEnv("TR_TRACKER_PROBE", "1")
+putEnv("TR_TRACKER_PROBE_PATH", probePath)
+if fileExists(probePath): removeFile(probePath)
+
+var bots = @[modularBotDir]
+for name in ["Fire", "Corners", "Crazy", "RamFire", "SpinBot"]:
+  let d = samplesDir / name
+  if dirExists(d): bots.add d
+if fileExists(drussDir / "DrussGT.sh"): bots.add drussDir
+
+echo "server jar : ", getEnv("TR_SERVER_JAR", DefaultServerJar)
+echo "probe path : ", probePath
+echo "reconcile  : ", getEnv("TR_TRACKER_RECONCILE")
+echo "bots       : ", bots
+echo "rounds     : ", rounds
+
+ensureServer()
+discard compileBots(@[modularBotDir])
+let raw = runBattleRunner(getServerUrl(), bots, rounds, 1_800_000, true)
+let r = parseServerOutput(raw)
+for b in r.results:
+  echo fmt"  {b.name:<12} totalScore={b.totalScore} rank={b.rank} survival={b.survivalCount}"
+echo "rounds played: ", r.rounds.len