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Phase 5 batch 2 (5b): VXISI / NaN handling for the FMA family. The 8 FMA opcodes (fmaddx/fmaddsx/fmsubx/fmsubsx/fnmaddx/fnmaddsx/fnmsubx/ fnmsubsx) all share two fix shapes: 1. VXISI on the add/sub step. The previous code passed `a*c` to check_invalid_add, which has separate rounding from the FMA. In extreme cases this gives the wrong sign (PPCBUG-202) or wrong infinity status. Worse, fmsub/fnmadd/fnmsub had NO add-step VXISI check at all (PPCBUG-181/182/203). The fnmsub pattern is the canonical Newton- Raphson step — the most common FPU path in Xbox 360 graphics code. 2. NaN sign preservation in fnmadd/fnmsub. ISA Book I §4.3.4 forbids negation of a NaN FMA result; Rust's unary `-` flips the IEEE-754 sign bit (PPCBUG-183/205). Fixes: - fpscr.rs: new helper `check_invalid_fma_add(ctx, a, c, b, sub)` that derives VXISI from input properties (mathematical-product sign + b sign) instead of from the lossy `a*c` value. Also covers SNaN. - interpreter.rs: all 8 FMA arms now use the new helper; fnmadd[s]/ fnmsub[s] gate the negation on `!fma.is_nan()`. Tests: - fmsub_inf_minus_inf_sets_vxisi: regression for PPCBUG-203. - fnmadd_nan_input_preserves_nan_sign: regression for PPCBUG-205. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>