Single-page arena ToT for all size-determinable construction (init_tiles_nested, retile)

[zhihao-deng]

Summary

Route the size-determinable arena ToT construction paths through the up-front
arena_outer_init allocator instead of the incremental, multi-page
ArenaToTBuilder, so every such outer tile is a single contiguous arena page.

Multi-page outer tiles silently disqualify the strided-BLAS outer-contract +
inner-AXPY fast path (it reverts to per-cell AXPY). make_nested_tile
(init_tiles_nested) and make_with_new_trange (TA::retile) had regressed to
the multi-page builder in 176df8a2, though their range/fill callbacks are
random-access — so the up-front allocator applies with no peak-memory penalty.

Why it matters

Fixing this at the construction layer makes the single-page guarantee universal:
all arena ToT built via these paths are single-page — not just one hand-patched
category. Downstream (e.g. MPQC CSV-CCk/PNO) every operand — amplitudes,
coefficients, energies, PNO/OSV-domain tensors — is single-page, instead of relying
on a post-hoc compaction of a single variable (which also costs an extra deep-copy
and a peak-memory spike). ArenaToTBuilder is kept for the one genuinely
single-pass path (init_tiles), preserving the 176df8a2 optimization where it
still applies.

Companion MPQC change (expected)

This TA change covers ToT built through init_tiles_nested/retile. MPQC also
constructs some operands via direct ArenaToTBuilder calls, which need a
matching conversion to arena_outer_init to be single-page — landing separately
in MPQC:

• cc/solvers.h: the PNO Jacobi amplitude update and the coefficient/energy
  accessors (_pno_coeffs_tot, _osv_coeffs_tot, f_pno_diag, osv_energy).
• mbpt/pao_to_pno_mp2.ipp: the PAO→PNO contribution-slice builders.
• mbpt/csv.ipp: drops the now-redundant compact_csv_coeffs post-hoc compaction
  (coefficients are now born single-page).

Validation

With both the TA and MPQC changes applied: new make_nested_tile/retile
single-page regression tests + full tot_construction suite pass. Downstream MPQC
CSV-CCk (C4H10) with TA_ASSERT_SINGLE_PAGE=1: 0 violations across 490,124 ToT
tiles, ~10% faster.

[evaleev]

Rebased onto current master (now includes the merged #571) and pushed a cleanup commit (35e96b527) addressing two review findings in the retile path (make_with_new_trange):

1. Duplicate cell walkthrough — arena_outer_init's range pre-walk now caches the source cell it finds per ordinal in src_cells, so the fill loop reuses it instead of re-running the (cached, but non-trivial) source_cell_at lookup. This restores the single-source-lookup-per-cell property the comment claimed; the stale comment is updated to describe the actual two-phase up-front build.
2. Redundant zero-init — pass zero_init=false to arena_outer_init, since every non-null cell is fully overwritten by the copy loop (matching the old ArenaToTBuilder default).

Verified: ta_test builds clean; tot_construction_suite passes at np=1. (The np=2 tot_construction_dist_suite SEGV in the MADNESS RMI worker thread reproduces identically on the un-modified branch — pre-existing ASan/comm-thread teardown artifact, not from this change.)