MP: peer accel/decel "snap" MEASURED ~0.64u (not ~2.9u) -- effectively resolved (task #50)

Went after the flagged ~2.9u peer-motion snap. Added a BT_SNAPLOG diagnostic
(mech4.cpp: logs the re-anchor drift eMag = |authority - dead-reckon| absorbed per
frame) and ran a controlled accel/decel soak: node B BT_AUTODRIVE + BT_DRIVE_SWEEP0
sweeping the throttle through STOP, node A observing, affinity-pinned.

Result: peer drift maxes at ~0.64u (median 0.54u, 18 events >0.5u over 48s), gently
absorbed at k~0.24/frame -- SUB-UNIT, at the noise floor. The ~2.9u figure was stale:
it predated the peer body-channel swap (96a896a, which put the peer on the same
channel the master's mirror predicts) AND was measured without the CPU-affinity fix
(packet-jitter-sparse records inflate the drift). The old "run the master mirror on a
leg-channel prediction" plan is moot now the peer is on the body channel.

No motion-code change -- just the measurement + the BT_SNAPLOG diagnostic (retained).
KB (multiplayer.md, open-questions.md) corrected to the measured value. checkctx CLEAN.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

game/reconstructed/mech4.cpp

@@ -2207,6 +2207,13 @@ void
 						const float eMag = (float)rmErr.Length();
 						float k = 0.15f + eMag * 0.15f;			// error-proportional catch-up
 						if (k > 0.6f) k = 0.6f;
+						// DIAG (BT_SNAPLOG, task #50): the peer drift = |authority - dead-reckon|
+						// absorbed this frame -- the "snap" magnitude.  Log > 0.5u so an accel/decel
+						// soak (BT_DRIVE_SWEEP0) measures the residual.
+						static const int s_snapLog = getenv("BT_SNAPLOG") ? 1 : 0;
+						if (s_snapLog && eMag > 0.5f)
+							DEBUG_STREAM << "[snap] drift=" << eMag << " k=" << k
+								<< " legAdv=" << replLegAdv << "\n" << std::flush;
 						// AT-REST snap (user "gliding stops"): when the legs have wound down
 						// (replLegAdv ~0), gliding the body to absorb the residual offset is
 						// VISIBLE because no leg motion masks it.  With the feet planted a