Backward step from the first two known stored poses (prev_pose/prev_ts
pre-initialized before the loop; the in-loop rate code is untouched): the
seed scene's rate becomes exactly (stored[next]-stored[first])/dt instead
of the zero-rate special case that left scene 0 with mb=(0,0) and the
uncompensated v*tau bias. Using a difference of actual poses avoids any
hand-derived rotation rate and its sign-convention risk. In RT production
the continuous stream always has a real previous frame; this seam also
covers stream (re)starts, where the nominal-rotation config can feed the
same prev_pose/prev_ts initialization. Deliberate consequence: the next
full-sequence run is NOT digit-identical to the 07/13 baseline in scene 0
(expected equal or better - the seed previously carried the v*tau bias).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>