refactor(weights): collapse compose_sample_weights to single-target API

refactor(weights): collapse compose_sample_weights to single-target API

LABEL_COLUMNS is single-target by design, so the dict-shaped per-label
map and row-wise aggregation in compose_sample_weights were dead
plumbing. Flatten the signature to a single label_weights vector and
read LABEL_COLUMNS[0] directly in _compose_per_row_weights. Drop the
duplicate-column guard (unreachable under single-target). Align caller
naming on base_weights to match the callee parameter. Add a defensive
check that LABEL_COLUMNS[0] is in dk.label_list to fail loudly if the
project label constant ever diverges from freqtrade's runtime view.