ForceFreeStates: eigenvalue cross-check diagnostic between integration paths

[logan-nc]

When both Route A (legacy Euler-Lagrange) and Route B (chunked-Riccati) are available — for example with the concurrent dual-route execution proposed in the companion issue — compare the first eigenvalue et[1] from each path on every run and emit:

• >1% relative discrepancy → @warn
• >10% relative discrepancy → @error

Write Route B's odet_riccati to HDF5 as a diagnostic so users can inspect the two estimates side by side.

Why: the cross-check is a cheap sanity check that has already caught real drift. On feature/forcefreestates-rewrite (PR #252), it flagged ~4.5% drift on a Solovev case that was otherwise silent. It also serves as a regression net while the chunked-Riccati path matures.

Reference implementation: cross-check logic in src/ForceFreeStates/Integration.jl on feature/forcefreestates-rewrite (PR #252).

Acceptance:

• Cross-check runs when both odet (Route A) and odet_riccati (Route B) are populated.
• Tolerances configurable but default to >1% warn / >10% error.
• odet_riccati written to HDF5 under a clearly named diagnostic group.
• One regression test exercising both the warn and error branches with synthetic eigenvalues.