Oct tree

[TobiBu]

oct-tree based implementation of FMM next to kd-tree and radix-tree based versions.

[Copilot]

Pull request overview

This PR adds an explicit octree execution backend path to jaccpot (currently scoped to basis="solidfmm"), wiring octree-native prepared-state artifacts into evaluation while keeping the existing radix-oriented default behavior.

Changes:

• Add execution_backend runtime policy (auto|radix|octree) and thread it through solver/runtime state, including prepared-state evaluation controls (jit_traversal override).
• Introduce octree-native scaffolding modules/adapters and extend nearfield plumbing to support explicit leaf/carrier particle-group ownership.
• Add extensive unit/integration test coverage and documentation updates for the octree backend workflow.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
jaccpot/config.py	Adds FMMExecutionBackend and exposes execution_backend in RuntimePolicyConfig.
jaccpot/solver.py	Threads execution backend into runtime and adds jit_traversal override for prepared-state eval.
jaccpot/runtime/_fmm_impl.py	Major wiring: stores execution backend in prepared state, builds octree artifacts, and passes nearfield interop / farfield overrides into evaluation.
jaccpot/runtime/_octree_adapter.py	Adds adapter to extract an explicit octree execution view from yggdrax trees.
jaccpot/runtime/_octree_fmm.py	New octree-native upward/downward scaffolding (M2L/L2L plans + helpers).
jaccpot/nearfield/near_field.py	Adds group-based scatter schedule prep + group-driven nearfield execution path.
jaccpot/upward/solidfmm_complex_tree_expansions.py	Adjusts level batching to avoid host/device introspection for JIT-safety.
tests/unit/test_solver_api.py	Adds solver-level tests validating backend selection, prepared-state eval, targets, JIT/eager parity, and cache behavior for octree backend.
tests/unit/test_octree_fmm_scaffolding.py	New unit tests for octree-native scaffolding (plans, multipoles, M2L/L2L consistency checks).
tests/integration/test_fmm.py	Relaxes a couple tolerances and adjusts x64 enabling + a nearfield policy test.
README.md	Documents octree backend usage + limitations.
docs/octree_fmm_task_list.md	Adds a task/status summary for octree backend.
examples/compare_yggdrax_jaccpot_prepare.py	Extends comparison example to include octree tree + radix vs octree execution backends.
OCTREE_JACCPOT_STATUS_2026-03-15.md	Adds a detailed status handoff document for ongoing octree work.

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