Track libneo release/26.2 and unify resolver

[krystophny]

Risk tier

• T0: docs, comments, small build metadata
• T1: pure refactor, no behavior change intended
• T2: local numerical logic
• T3: physics, output behavior, coordinate convention
• T4: parallelism, GPU, dependency, CI, or security-sensitive build logic

Correctness contract

Intended behavior change

Pin the default libneo fetch branch to the release/26.2 line for validation, unless LIBNEO_BRANCH is supplied by CMake or the environment.

Behavior that must not change

Explicit LIBNEO_BRANCH overrides still win. Before this PR merges, the default must be changed from the release branch to the released tag 26.2.0.

Coordinate / unit conventions

Unchanged.

Numerical invariants

Unchanged. This is a dependency pin only.

Tests added

• unit: none
• integration: existing Boozer chartmap tests exercised
• system: none
• golden record: unchanged

Golden-record impact

• unchanged
• changed

Failure modes considered

• Accidentally pinning main or a moving branch on SIMPLE main.
• Blocking local/CI overrides that deliberately test another libneo branch.
• Merging before the libneo release tag exists.

Manual validation

The branch remains draft. It should not merge until release/26.2 is replaced by tag 26.2.0.

Verification

make test-nopy TEST=test_boozer_chartmap VERBOSE=1
...
1/3 Test #19: generate_test_boozer_chartmap_data ...   Passed    0.06 sec
2/3 Test #20: test_boozer_chartmap .................   Passed    0.14 sec
3/3 Test #23: test_boozer_chartmap_roundtrip .......   Passed    5.68 sec
100% tests passed, 0 tests failed out of 3

[krystophny]

@krystophny heads-up: this PR is part of a cross-repo change unifying how the codes resolve libneo (a validated LIBNEO_REF override and tracking the release/26.2 branch by default), plus an external CI trigger so the libneo release gate can build this code against a candidate ref. It needed a few more changes than expected, so a review before it lands would help. We plan to merge the whole set on Monday, 2026-06-15. Companion CI PR: #367.

[krystophny]

Superseded by the new release model (no release branches / versions): SIMPLE floats on libneo main and is validated per libneo PR via the reverse-dependency gate. The release/26.2 pin and resolver rewrite here are no longer the direction. CI wiring lives in #367 (merged) + #369. See the 'Release process 2026' thread.