fpnew: Fix PULP DivSqrt RDN/RUP rounding, document unsupported RMM - #179
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DivSqrtSel=PULP forwarded RISC-V rounding-mode encoding directly into div_sqrt_top_mvp's RM_SI, but the two encodings disagree for RDN(2)/RUP(3), causing results to round in the wrong direction. Add the missing translation in fpnew_divsqrt_multi.sv.
RMM has no equivalent in div_sqrt_top_mvp at all (only ties-to-even is implemented) and can't be fixed by translation alone; add a $warning in fpnew_opgroup_multifmt_slice.sv documenting this, matching the existing FP8/THMULTI notice pattern.
Partially fixes #178 (RDN/RUP); RMM is a separate, deeper issue (missing hardware support in div_sqrt_top_mvp itself) and is intentionally left unaddressed here.