Previously segment 0 only filled positions [0:895], leaving [896:1023]
as zeros from the initial block. These zeros propagated into the overlap
region of subsequent segments, corrupting the convolution.
Fix: change buffer-full threshold from SEGMENT_ADVANCE (896) to
BUFFER_SIZE (1024). Add zero-padding for the last segment when chirp
data runs out before the buffer is full. Updated testbench accordingly.
Verified: multi-segment 32/32 PASS, integration test 10/10 PASS.
17b70bdcff