WIP: CPU network SoA dual-write (not for merge)

[nrnhines]

Status: Work in progress — do not merge to master

Exploratory stack for CoreNEURON-shaped network data on the NEURON CPU side. Intended as a durable review/reference branch, not a landable feature PR.

Planned follow-on branch (later): hines-grok/cpu-net-soa-heap-free (or similar), forked from this work after dual-write+sort, to drop long-lived NetCon::weight_ under a defined ABI plan.

GPU native (later, undetermined order relative to master): spike queue + threshold → net_send buffer on GPU; NET_RECEIVE net_send on GPU; ARTIFICIAL_CELL net_send stays CPU; distribute via net_receive buffer. Whether heap-free lands on master before that GPU track is undecided.

What this PR contains

• Phases 0–4 dual-write: PointProcess, Weight, NetCon, PreSyn SoA; SelfEvent weight_index
• Network containers in nrn_ensure_model_data_are_sorted (thread packing, weight-block contiguity, fanout rebuild)
• SaveState / BBSaveState dual-write (SoA values + NetCon-index SelfEvent identity)
• Developer docs: doc/network-soa/ (topology, heap policy, sort) + GROK-NETWORK-SOA.md + doc/network-soa-phase0.md

Explicitly not in this PR

• Freeing NetCon::weight_ (nocmodl still needs long-lived MOD scratch for net_send(..., _w) / FOR_NETCONS / INITIAL)
• GPU net buffers / native GPU network
• Thin remote InputPreSyn / gid→fanout redesign
• Performance claims vs NEURON 8.2.7 or CoreNEURON (benchmark plan to be separate)

Docs for reviewers

Start at doc/network-soa/README.md.

Test plan

• Full ctest -j 8 on author machine (post-regression fixes)
• CI (when draft is marked ready or CI runs on draft)
• Future: ringtest / space / time vs NEURON 8.2.7, this branch, CoreNEURON (linux + Apple M1)