fix(quickadapter): make causal k-NN weight availability exact (#151)

fix(quickadapter): make causal k-NN weight availability exact (#151)

Make the causal availability of the label-WEIGHT column exact for adaptive
k-NN Gaussian-fill bandwidths. Compute production 1D k-th-neighbor distances in
O(M log M), and propagate the earliest complete confirmation group at which each
clipped k-NN sigma is provably stable, over the confirmable Zigzag suffix
geometry: a confirmed pivot needs at least five slope observations, so
successive pivots are at least six candles apart and the last future candidate
is n-6. Initial-orientation replay is handled atomically via a position-only
successor bound (last replayed pivot + 6); later groups use the exact
confirmation frontier. Floor/ceiling/interior/singleton stability is decided on
the effective rank min(k, pivot_count-1) jointly with that geometry.

Truncate causal Gaussian fills to the finite support ceil(4*fill_sigma_candles)
tracked by availability; causal_mode=false keeps the legacy dense unbounded
Gaussian bit-identical. Pure-Gaussian uniform pivot centers keep their own label
availability (fixed, constant-clipped, adaptive); only adaptive off-center bands
additionally wait for sigma availability. Thread each label column's
weighting_config to availability.

Add the first quickadapter test harness, locking the Zigzag geometry invariants
the availability proof relies on (min pivot spacing, first-future bound, k-NN
availability bounds), runnable in the freqtrade container.

Fixes #131