riscv: avoid FMA error recovery on special operands

[carlosqwqqwq]

riscv: avoid FMA error recovery on special operands

Fixes #306

Commit message

riscv: avoid FMA error recovery on special operands

Description

The software FMA error-recovery path applies finite-value error-bit manipulation to infinity and NaN cases, producing invalid payloads and spurious exception flags. Classify the special operands before the finite error-recovery logic. The change is limited to src/util/fpu_lib.h; validation artifacts stay out of the source diff. The patch applies independently to the staging branch.

Validation

• The witness runs on native RISC-V hardware, QEMU, and the affected RVVM build; the patched build matches both references.
• Both the interpreter and JIT lanes were exercised, and the patched build is limited to this root.