When music repeats itself, a recurrence plot shows the repeat as bright
blocks away from the diagonal — here a planted A-B-A pattern lights up
exactly where it should, and the section boundaries come back exact.
The input is a 60-frame, 12-dim feature matrix planted as
A(20)-B(20)-A(20) with small
deterministic jitter. recurrenceMatrix(mode 'affinity', sym)
must light the A↔A repeat as bright off-diagonal blocks, keep the
|i−j|<width band exactly zero and the matrix exactly symmetric; the lag
shear must round-trip bit-exactly; agglomerative(data, 3) must
return exactly [0, 20, 40]. Same asserts as
examples/node/segment-recurrence.mjs (node-verified).