New corrections

[rodrigoaramendes]

PCCD/AP1roG wavefunction with memoized seniority filtering and per-parameter-vector overlap caching.

The assignment methods (assign_nelec, assign_ranks, assign_exops, assign_refwfn, assign_s_type) are carried over unchanged from the reference PCCD implementation; only the overlap routines differ.

Differences from the "old" PCCD implementation

1. The three near-identical seniority-filter blocks are factored into a single _apply_seniority_filter helper. Pure deduplication; no change in results.

2. The filter is memoized. It is idempotent (rerunning it on an already-filtered array reselects the same rows) and upstream mutates exop_combinations[key] in place, so re-deriving the selection on every call is wasted work. Two separate key sets are kept because _olp filters against occ_indices(sd2) (the reference) while _olp_deriv filters against occ_indices(sd) (the target). generate_possible_exops is overridden so that if BaseCC's refresh_exops mechanism discards exop_combinations, the filter caches are dropped with it. Without that, a regenerated (unfiltered) key would be treated as already filtered.

3. _olp(sd) / _olp_deriv(sd) are memoized per parameter vector. For fixed amplitudes these are pure functions of sd, and the same sd recurs many times within one objective or gradient evaluation (once as a projection-space element, again as an excitation neighbour of other elements inside integrate_sd_wfn). The caches are cleared from assign_params, assign_refwfn and clear_cache, so a cached value is only reused while the parameters that produced it are current.

4. legacy_deriv_filter controls a genuine behavioural divergence. The reference _olp_deriv and _olp_double_derivative apply the "sen-o" filter unconditionally, ignoring s_type (only _olp dispatches on it). With legacy_deriv_filter = True (the default here), that behaviour is reproduced exactly. With False, the derivative honours s_type, which makes the gradient the actual derivative of the value this object returns, relevant only for s_type != "sen-o", since the two agree on the default.