Improve Symmray MPS sampling strategy

[rezaquant]

What changed

• Added a public strategy= alias for MpsSampler while preserving prefix_strategy=.
• Added budget-aware automatic dense batching for resolved fermionic U1/U1U1 Symmray MPSs.
• Kept parity-collapsed Z2/Z2Z2 states on the charge-aware sparse path.
• Added dense-memory diagnostics, documentation, and sampler regression tests.

Why

High-entropy Symmray batches were falling back to many serial prefix contractions. The dense native batch kernel is substantially faster when the estimated dense MPS view fits the configured memory budget.

Validation

• 83 sampler tests passed.
• 31 public API/package tests passed.
• Ruff passed for the changed Pepsy files.