ForceFreeStates - REFACTOR - ⏸️ Single-source the auto chunk-count target

[d-burg]

Release note

• Audience: developers
• Numerical impact: none (harness @ a47f409) — the resolved chunk targets are unchanged; diiid_n1_riccati is bit-identical head vs base
• Migration: none

The nchunks = 0 auto chunk-count target and the crossing floor were each written out in three places: the authoritative formula inside balance_integration_chunks plus hand-copied mirrors in runtests_parallel_integration.jl and runtests_decomposition_invariance.jl. They are now single functions — ForceFreeStates.auto_chunk_target(msing) and ForceFreeStates.min_crossing_chunks(msing) — used by the solver and both test files, and the struct docstring, docs/src/stability.md and the test comments now point at them instead of restating the formula.

Regression report

regress --cases diiid_n1_riccati --refs 30eb52073,a47f409f0 --force (base = #362's head feature/thread-invariance, so the comparison isolates exactly this refactor; raw shas, both sides fresh):

Regression Report: diiid_n1_riccati
Ref 1: 30eb52073  @ 30eb52073 (2026-08-28)
Ref 2: a47f409f0  @ a47f409f0 (2026-08-28)
Quantity                       30eb52073      a47f409f0      Diff     Status
delta prime (BVP diagonal)     [5 elem]       [5 elem]       0.0e+00  OK
delta prime (raw side-major)   [10 elem]      [10 elem]      0.0e+00  OK
edge coil response delta_coil  [10 elem]      [10 elem]      0.0e+00  OK
total energy Re(et[1])         8.038607e-01   8.038607e-01   0.0e+00  OK
plasma energy Re(ep[1])        -1.344628e+00  -1.344628e+00  0.0e+00  OK
vacuum energy Re(ev[1])        2.148488e+00   2.148488e+00   0.0e+00  OK
total energy (all)             [35 elem]      [35 elem]      0.0e+00  OK
# singular surfaces            5              5              0.0e+00  OK
singular psi locations         [5 elem]       [5 elem]       0.0e+00  OK
singular q values              [5 elem]       [5 elem]       0.0e+00  OK
ODE steps (saved)              51             51             0.0e+00  OK
ODE steps (total)              1667           1667           0.0e+00  OK
mpert                          35             35             0.0e+00  OK
npert                          1              1              0.0e+00  OK
q0                             1.204212e+00   1.204212e+00   0.0e+00  OK
q95                            4.781723e+00   4.781723e+00   0.0e+00  OK
beta_n                         1.372511e+00   1.372511e+00   0.0e+00  OK
Runtime (s)                    107.8s         107.8s                  --
Summary: 17 unchanged

Validation

• runtests_parallel_integration.jl at a47f409f0, julia -t 4, fresh worktree: 95/95
• runtests_decomposition_invariance.jl same conditions: 11/11
• The test workflow does not trigger while the base is feature/thread-invariance (it filters on base develop/main); it will run once the base is retargeted after Test - TEST - ⏸️ Decomposition invariance of the unified Riccati Delta-prime path #362 merges — the local runs above cover the gap

Notes for reviewers

• The formula is pinned by value in runtests_parallel_integration.jl: auto_chunk_target.((0, 1, 5)) == (8, 17, 53) and min_crossing_chunks.((0, 1, 5)) == (3, 5, 13). A change to the src formula fails that test and has to update a spec, rather than being pasted into a copy of the expression. (The harness diiid_n1_riccati case would also catch it through the chunk list.)
• auto_chunk_target drops the max(2m+3, …) from the old expression: 9m+8 > 2m+3 for every m ≥ 0, so the floor could never win. Value-identical for every msing.
• The min_bvp_intervals conditioning rationale moved from the inline comment into auto_chunk_target's docstring, which also corrects the old comment's stale "at least 5 non-crossing sub-chunks" to the 8 the code uses.
• Tests after these edits (b7ace7893): runtests_parallel_integration.jl 96/96.
• Stacked on Test - TEST - ⏸️ Decomposition invariance of the unified Riccati Delta-prime path #362 (feature/thread-invariance), since runtests_decomposition_invariance.jl only exists there; retarget to develop after Test - TEST - ⏸️ Decomposition invariance of the unified Riccati Delta-prime path #362 merges.

---

Draft until #362 lands and Daniel names reviewer/assignee.

🤖 Generated with Claude Code

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[logan-nc]

@d-burg should this be opened and assigned a reviewer?