#45 scoreboard: reclaim the binary's DEATHS field, add a heartbeat, sweep the false KB claims

Follow-up to 4fa7eee, driven by an adversarial review pass and three rig cycles.
Each item below is a real defect that pass found, not polish.

DEATHS now uses the binary's own column.  PilotList::Execute @0x4cabd0 draws
`fild [edi+0x27c]` (KILLS) and `fild [edi+0x280]` (DEATHS); +0x280 has exactly
two runtime writers image-wide plus the ctor zero.  Our port had declared it
`pad_0x280`, never written, and displayed the ENGINE's Player::deathCount
(+0x200) instead -- which is the respawn-handshake identity, seeded -2, and is
why it needed a clamp to pass as a count.  It is now BTPlayer::deathTally (our
offset 0x274, offsetof-locked), incremented beside ++deathCount, read by the
gauge, and replicated.  deathCount is left to the engine's handshake.  So this
half moves TOWARD the binary; it also closes the plan's Headline-1.
(`deathTally`, NOT `deaths`/`deathCount` -- those would shadow the base.)

The mirror was not self-healing: update records are UNRELIABLE by construction
(Entity::UpdateMessage clears ReliableFlag, ENTITY3.h:112; the relay's UDP path
also drops stale/reordered datagrams), and the pair was dirtied only on an
event.  One lost datagram left every peer stale until that pilot's next kill or
death -- forever for the last kill of a round.  Added a 2s heartbeat that
re-dirties the record, which also bounds how long the binary's phantom
partner-increment stays visible.  The timer is a function static deliberately: a
data member would change sizeof(BTPlayer) and break the offset locks.

Also fixed: the SBMIRROR row AND its change-detect both still read deathCount (a
constant -2 here), so the "log only the edge" guard could never be false and
every row printed deaths=-2.  That is what made the first rig runs look like
DEATHS was broken when a WRITE/READ trace proved the transport correct.  Row
count per node fell from ~20 to 3, one per real change.

Guarded the record against per-bit layouts: update_model is a BIT INDEX and
Entity::WriteUpdateRecord switches on it (ENTITY.cpp:329-352) -- the DamageZone
bit emits a variable-length packed stream whose length it computes itself, so
appending two ints and re-stamping recordLength over that would corrupt it.

Deleted BTPlayerCountObservedDeath -- definition, call site, extern and friend
together, since /FORCE hides stragglers.  It never executed (its call site sat
inside the once-per-death transition, which a replicant never enters) and could
not have worked (0 of 8800 corpus DMG rows target a replicant, which is why every
DEATH inst=R row reads killer=0:0).  Under replication it would have been a
second writer of a replicated counter.

New forensics: NOCREDIT names the failing link when a kill credit is skipped (it
used to be completely silent -- the counter simply never moved), and PLAYER_LINK
records whether the one-shot link resolved.  Both retire the NULL-playerLink
theory: every rig shows `PLAYER_LINK inst=R resolved=1` and no NOCREDIT rows.
PLAYER_DEAD now logs both counters (deaths=handshake, tally=scoreboard).

KB sweep of the claims that hid this bug for so long:
  * context/gauges-hud.md's "RESOLVED -- deaths tally per node from locally
    observed events" was FALSE; corrected with the measured evidence.
  * docs/GAUGE_COMPOSITE.md + btl4gau3.cpp "the dead pad_0x280" -- never dead,
    merely unwritten.
  * btplayer.hpp attributed VehicleDeadMessageHandler to @004c012c; it is
    @004c05c4 (absent from the decomp export -- the #60 gap).
  * docs/RESPAWN_REARM_PLAN.md's "#45 SUBSUMED" -- the PLAYER_DEAD symptom was
    subsumed, the scoreboard defect was not.
  * The corpus figure in the record banner now states its method so it is
    reproducible.

Rig-verified (2-node loopback, several cycles): owner 3:1 kills 2/tally 1 read
`kills=2 deaths=1` on the peer; owner 2:1 kills 1/tally 2 read `kills=1 deaths=2`
on the peer.  Respawn unaffected, no crash, no GLITCH rows.  Still awaiting live
multi-pod verification by a human; all pods must run the same build.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/GAUGE_COMPOSITE.md

@@ -528,8 +528,10 @@ a prose-only vtable slot AVs on first call).
   ValveSetting→coolantFlowScale@0x15C. The SET valve indicator renders (row=Round(span*value)).
 - ✅ **PilotList** (Comm KILLS/DEATHS roster; part_014.c:3156-3434, DAT_0051af88 8×3 layout PE-parsed) —
   the Comm surface now renders the live local-pilot row (KILLS 0 DEATHS 0 + name box). Feed = new
-  `BTResolveRosterPilot` bridge (mechmppr.cpp) → mapper roster. DEATHS redirected from the dead pad_0x280 to
-  the real deathCount@0x200. The scoring feed that moves KILLS/DEATHS is P3.
+  `BTResolveRosterPilot` bridge (mechmppr.cpp) → mapper roster. DEATHS now reads the binary's own
+  `+0x280` column (`BTPlayer::deathTally`, our offset 0x274) -- **corrected 2026-07-25 (#45)**: it was
+  briefly redirected to `Player::deathCount@0x200`, but that field is the respawn-handshake identity
+  (seeded -2), which is why it needed a display clamp. `+0x280` was never dead, merely unwritten. The scoring feed that moves KILLS/DEATHS is P3.
 - ✅ **GeneratorCluster** (4 generator panels, buttons 9-12) — the 4 panels (A/B/C/D) render with blue
   OutputVoltage bars (the @004c72ac ScalarBarGauge Scalar* variant + the Generator OutputVoltage table),
   labels, and status lamps. **Root cause of the initial abort (cdb-attach to the frozen dialog):** the engine