Add input validation for gravitational constants in compute_scales

[Copilot]

The compute_scales function had defensive logic for G_new == 0 in the consistency check but no upfront validation, allowing physically invalid values to propagate through calculations.

Changes

• Input validation: Check G_new and G_old are positive at function entry, raising ValueError with physically-motivated error message
• Simplified consistency check: Removed ternary operator (abs(G_new) if G_new != 0 else 1.0) since validation guarantees G_new > 0

def compute_scales(G_old, G_new, s_L, s_V, s_M):
    # Validate G_new is physically meaningful
    if G_new <= 0:
        raise ValueError(f"G_new must be positive (got {G_new}). Zero or negative gravitational constants are not physically meaningful.")
    if G_old <= 0:
        raise ValueError(f"G_old must be positive (got {G_old}). Zero or negative gravitational constants are not physically meaningful.")
    # ... rest of function

This prevents division-by-zero and provides clear error messages for invalid inputs rather than confusing mathematical errors downstream.

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[The9Cat]

Yes, looks good.