Diag: add BT_SHOT backbuffer dump + correct/park the -net render glitch (task #53)

Scouted the "-net node render glitch". Two prior framings were wrong and are
corrected in the KB: it is NOT device contention (retracted earlier), and it is
NOT a from-load glitch -- the trigger is a RESPAWN (user correction). It's a
post-respawn render-rebuild fault on the MP path.

Could not reproduce in 9 automated 2-node loads + a forced death/respawn (all
clean). Debug uninit is a deterministic 0xCDCDCDCD, so the intermittency implies
a race -- and the task #52 respawn fixes (LoadMission re-entry, viewpoint relink,
death-latch) plausibly already closed it. Parked as WATCH; re-open if it recurs.

Tooling (general, off by default): BT_SHOT=<path> dumps a per-instance backbuffer
PNG from the world-render path (L4VIDEO.cpp) every 90 frames -- lets us capture a
specific node's frame without foregrounding the window. Used with
scratchpad/hunt53.sh (N-run divergence hunter) to verify a fix if the glitch
returns.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

context/multiplayer.md

@@ -37,19 +37,30 @@ all connect. The **CONSOLE** (operator station — ABSENT from every archive) de
 emulator** (⚠ `NotationFile::ReadText` expects NUL-SEPARATED lines). [T2]
 
 ## Verified milestones (one box, two instances)
-- ⚠ **`-net` NODE RENDER GLITCH — OPEN (observed live 2026-07-09, cause NOT yet found):** in a 2-node
-  session ONE window comes up visually GLITCHED FROM LOAD (before any death) — shadow clipping, camera
-  stuck inside/blocked, targeting/firing feels dead — while the OTHER is fine. **Isolated by test, not
-  theory:** (a) a SOLO instance is perfect; (b) **TWO SOLO instances (no `-net`) are BOTH perfect**
-  (each logs `MECH under boresight … IN WEAPON RANGE`, no device fallback/error, both hardware HAL
-  devices). So it is **NOT** a two-D3D9-apps-on-one-GPU / device-contention issue (an earlier note
-  claimed that — WRONG, retracted) and NOT the warp/respawn (from-load, pre-death). It is **specific
-  to the MP `-net` path** — the console-driven mission-start ladder / replication doing something to
-  ONE node's render+input state, asymmetrically (the two nodes differ in launch order, affinity,
-  net port, BT_MP_NET, BT_MP_FORCE_DMG). CAUSE UNKNOWN — next step: determine WHICH node glitches
-  (A@1501 vs B@1601) and correlate, then audit the MP viewpoint/camera/HUD setup vs the (clean) solo
-  path. Does NOT affect the pod (one instance per machine). Dev workflow: trust the SOLO window for
-  visual/gameplay checks; the 2-node logs are still trustworthy for replication/netcode logic.
+- ⚠ **`-net` node render glitch — LIKELY ALREADY FIXED / WATCHING (task #53):** a 2-node MP session
+  once showed ONE window come up visually wrong — shadow clipping, camera stuck inside/blocked,
+  targeting/firing dead — while the other was fine. Two earlier framings were WRONG and are corrected:
+  (i) the "two-D3D9-apps-on-one-GPU / device contention" theory was retracted — a solo instance, AND
+  two solo instances, are all clean, so it is not device contention; (ii) "glitched FROM LOAD, before
+  any death" was ALSO wrong — **the trigger is a RESPAWN** (user correction, 2026-07-10). So it is a
+  **post-respawn render-rebuild fault on the MP path**, not a load-time one. Does NOT affect the pod
+  (one instance per machine).
+  - **What the respawn rebuild does (reproduced 2026-07-10):** on respawn the player's mech is rebuilt
+    via `MakeEntityRenderables`, which runs a TWO-STEP view sequence — inside skeleton + `COCKPIT
+    eyepoint`, then outside skeleton + `external chase` (`[view] …` logs, btl4vid.cpp:2158) — and the
+    external chase camera is NOT re-installed (only ONE `[BTrender] external debug chase camera
+    installed`, btl4vid.cpp:648, for the whole session despite multiple respawns). Plausible failure:
+    if that two-step completes only to the COCKPIT step (raced/interrupted on the `-net` ladder), the
+    node is left stuck INSIDE — body hidden by the inside skeleton, shadow/reticle flagging with it =
+    the reported bundle. Camera + reticle + view + `tshd` shadow are all gated on `buildDebugChaseCamera
+    = (view_type == insideEntity)` (btl4vid.cpp:297).
+  - **Status: could NOT reproduce (2026-07-10) → likely already fixed.** 9 automated 2-node loads PLUS a
+    forced death/respawn all came up CLEAN (the respawn completed to external chase; clean backbuffer via
+    the new `BT_SHOT` dump). Since debug uninit is a deterministic `0xCDCDCDCD` (would glitch every run),
+    the intermittency implies a **race**, and the task #52 respawn fixes (the `LoadMission` re-entry
+    killer bug, viewpoint relink, death-latch) plausibly ALREADY closed it. **Left as WATCH** — re-open
+    and capture with `BT_SHOT` if it recurs. Tools kept: **`BT_SHOT=<path>`** (per-instance backbuffer
+    PNG, off by default, L4VIDEO.cpp) + `scratchpad/hunt53.sh` (N-run divergence hunter).
 - **P6 smoke test:** two instances share a world — console egg → TCP mesh → synchronized start →
   bidirectional entity replication (each renders its own mech + the peer's REPLICANT), 0 crashes.
 - **Movement replication:** A's mech WALKS on B's screen (the replicant tracks the master to ~1.5u via