Clarify [Rs, Rr] time-sign semantics and restore correct QMD/IMD freedom for improper-operation inputs

[Copilot]

The issue was caused by ambiguous time-reversal handling for [Rs, Rr] transformations: determinant-based inference was over-constraining QMD/IMD for mixed improper operations. For the reported input class, this collapsed expected tensor freedoms (QMD 4 / IMD 3) into incorrect lower-dimensional solutions.

• Core parser semantics

  • Updated parse_transformation() so [Rs, Rr] no longer infers anti-unitary/time-reversal sign.
  • For [Rs, Rr], s_T is now fixed to +1; anti-unitary operations are represented explicitly via [Tflag, Rr].
  • Kept spatial/parity handling unchanged (Rr drives axis transform and s_P via det(Rr)^P_constraint).

• Behavioral alignment in rank-3 convenience paths

  • This removes accidental time-sign flips for improper [Rs, Rr] inputs used by solve_qmd() / solve_imd().
  • Reported class now yields the expected freedom counts and z-sector relations for QMD/IMD.

• Targeted test updates

  • Added regression coverage for the reported improper-operation QMD/IMD scenario (expects 4 and 3 independent components respectively).
  • Migrated anti-unitary fixtures (AHE/31m cases) to explicit [Tflag, Rr] representation.
  • Added a small fixture helper to convert [Rs, Rr] -> [Tflag, Rr] where needed, reducing repeated conversion logic.

# Explicit anti-unitary representation (preferred when time-reversal is intended)
transformations = [
    [False, Rr_unitary],   # unitary
    [True,  Rr_antiunit],  # anti-unitary
]

A, ns, rel, comp = solve_qmd(transformations, tol=1e-5)

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