ForceFreeStates: file split (DeltaPrime/Chunking) and unified integration_method API #255

Two related cleanups carried by the historical-record PR #252 that are worth landing on top of #178.

File split

Extract from the current combined files:

• DeltaPrime.jl — Δ' matrix assembly (≈817 lines on the rewrite branch).
• Chunking.jl — cost-based load-balanced partitioning, including the empirical log-divergent cost model from Glasser 2018 / STRIDE (≈202 lines).

This makes Riccati.jl / Integration.jl smaller and gives Δ' assembly and chunking their own units of testability and review.

Unified integration_method / integration_threads API

Collapse the per-feature flags (use_parallel, use_riccati, parallel_threads) on PerturbedEquilibriumControl into a single dispatcher:

• integration_method::Symbol — e.g. :ChunkedRiccati (default), :LegacyEulerLagrange, plus whatever follow-on names make sense.
• integration_threads::Int — threads available to the dispatched method.

Pre-0.1.0 we don't need deprecation shims for the old flags — see the project memory note no-deprecation-shims-pre-010. Update the example gpec.toml files and any tests that still reference the old flags.

Reference implementation: src/ForceFreeStates/{Integration,Riccati,DeltaPrime,Chunking}.jl on feature/forcefreestates-rewrite (PR #252).

Acceptance:

• DeltaPrime.jl and Chunking.jl exist as their own files with focused responsibilities.
• use_parallel / use_riccati / parallel_threads removed from PerturbedEquilibriumControl; only integration_method + integration_threads remain.
• All example TOMLs and tests updated.