refactor: take the tree options as one group (F09: 45 → 40 parameters)

[TobiBu]

Third group of F09's interface half: 45 → 40 parameters. Branched off main, not stacked.

The parity check finally caught something

TreeConfig.tree_type defaults to None; the constructor defaulted to "radix".

A straight swap would have handed every caller who omits the group tree_type=None — silent,
and not something any test in the suite asserts directly. The golden wouldn't have caught it
either, since its tree cases pass an explicit mode.

Resolved at the unpack with the same or "radix" the facade already applies at its own
construction call:

tree_type = "radix" if _tree.tree_type is None else _tree.tree_type

Three groups in, one mismatch, and it was on the third. That's the argument for running the
check every time rather than after being surprised by one.

This mapping is also the widest so far — mode for tree_build_mode, leaf_target for
target_leaf_particles — so it's written out at the unpack and in the docstring, and asserted
complete with no TreeConfig field left unmapped.

A mistake worth recording

Converting dict(...) builders is only correct when the dict feeds an engine construction.
My sweep converted every dict(...) carrying a matching key, which also hit three feeding
derive_split_build_default, resolve_dehnen_geometry, and the facade (FastMultipoleMethod,
which takes flat kwargs) — 9 unit failures, all surfacing after the golden had already passed.

Reverted those three. The rule for the last group: check the consumer, not just the keyword.

That's now twice this refactor that the golden went green while call sites were broken. It
verifies resolved state; it says nothing about whether every caller still constructs.

Verification

pre-commit run --all-files     black / isort / pydoclint / flake8 all Passed
JAX_ENABLE_X64=1 pytest        1521 passed, 119 skipped
pytest tests/characterization  34 passed — golden data file unmodified
JACCPOT_RUNTIME_TYPECHECK=1 pytest tests/unit   1264 passed, 52 skipped

Remaining

FarFieldConfig — 9 params across 39 sites, the last group and the largest. 63 → 40 so far;
that one would take it to ~32.

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