FFS - REFACTOR - Reorganize ForceFreeStates into subdirectories (pure-move PR) #394

Motivation

ForceFreeStates has grown organically: Riccati.jl is a ~2000-line monolith mixing four distinct concerns, the singular-surface machinery is scattered across files, and ForceFreeStatesStructs.jl is an everything-bucket of types owned by different subsystems. Galerkin already shows the target pattern (its own subdirectory); the rest of the module should follow.

Proposed layout

• Riccati/ — split the monolith along its natural seams:
  • Propagators.jl — FM propagator integration (chunk integration, bidirectional crossing chunks)
  • DeltaPrimeBVP.jl — Δ′ BVP assembly + solve + PEST-3 decomposition
  • Crossings.jl — singular-surface crossing algorithms
  • Driver.jl — chunk orchestration (riccati_eulerlagrange_integration)
• Surfaces/ — singular-surface finding/filtering and GGJ coefficients in one place: sing_lim!, sing_find!, sing_min!, resist_eval_all!, SingType
• Matching/ — new home for inner-layer matching work (delta_mn, ResistiveMatch models; see REFACTOR_PLAN.md D13/D14) rather than bolting it onto Riccati or Galerkin
• Structs colocated with owners — dissolve ForceFreeStatesStructs.jl so each type lives next to the code that produces it

Exact file boundaries to be finalized when the PR is cut; the list above is the agreed shape.

Process constraints (agreed, non-negotiable)

• Pure-move PR: zero logic changes, so the whole diff is verifiable with git diff --color-moved=dimmed-zebra --color-moved-ws=allow-indentation-change --histogram plus a byte-identity HDF5 check.
• Sequenced after the interface PR (FFS - REFACTOR - Publish solves as ForceFreeStatesResult; staged main; solve API (refactor plan interface PR) #393), GPEC - IMPROVEMENT - Make all input control structs immutable #367, and the repo-wide formatter PR — moves must not collide with in-flight content changes.
• Never folded into feature commits.

Verification

• Full test suite unchanged
• Byte-identical gpec.h5 on a forward and a riccati deck vs the pre-move commit
• Regression harness: no tracked quantity moves

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