docs: the SoC under a full core, measured; the heatsink question closes

CAMPAIGN-LOG gains the 2026-08-24 drill: five minutes of openssl speed on
the one core over the live camera stream, 30 C chassis, plateau 70.8 C
with no throttle and every cooling device at state 0. BRINGUP's facts bank
drops its open question (whether ForgeFIRM's load wants the heatsink the
factory never fitted) for the measured fact: 14 C of headroom to the
passive trip under full load, a hot-chassis case above roughly 44 C.

Docs only, outside every layer; no acceptance consequence.

docs/BRINGUP.md

@@ -714,9 +714,13 @@ is committed.
   both GPU cooling devices; the critical trip is the kernel's orderly
   poweroff. A throttle slows the engine, the camera and the protocol thread
   before the step stream (the ring is in hand). The factory board carries
-  **no heatsink or fan on the SoC**, only the mounting holes for one; the
-  factory's load never needed it, and ForgeFIRM's may, which is what the
-  per-job SoC range and the throttle log line are there to show.
+  **no heatsink or fan on the SoC**, only the mounting holes for one, and
+  needs none: under a full core for five minutes on top of the live camera
+  stream, in a 30 C chassis, the bare die plateaus at **70.8 C**, 14 C under
+  the passive trip, with the core at 996 MHz and no cooling device off
+  state 0. The die-to-chassis delta at full load is about 41 C, so the
+  passive trip is a hot-chassis case (above roughly 44 C), not a load case.
+  The per-job SoC range and the throttle log line are the running record.
 - **Fan speeds at the cut profile** (exhaust duty 65535, intake 43278, air
   assist 1023; sampled at 1 Hz over 120 s from idle, the exhaust duct's
   inline booster fan off): exhaust **11640 rpm** steady (spread 11444 to