unprivileged/integrated-matrix: Refactor SAIL pseudocode into shared helpers

Extract common patterns from the 11 GEMM instruction SAIL blocks into
5 shared helper functions, eliminating ~300 lines of duplication:

  decode_gemm_geometry(W, vl_divisor_is_lambda) — unified preamble
    that decodes SEW, LMUL, lambda, computes K_eff/M/N/MUL_C, and
    checks VL divisibility and MUL_C legality.  Parameterised by the
    widening factor W and the VL divisor convention (K_eff vs lambda).

  read_block_scales(vm, i, j, s, R, EEW_C, fmt_C, rm) — reads and
    unpacks paired E8M0 block scales from v0, returning the combined
    scale and a NaN flag.  When vm=1, returns (1.0, false).

  fp_block_dot(i, j, k_lo, k_hi, g, fmt_A, fmt_B, fmt_C, rm, vs1, vs2)
    — FP inner product over a K-dimension block with widening.

  int_block_dot(i, j, k_lo, k_hi, g, signed_A, signed_B, vs1, vs2)
    — exact signed/unsigned integer inner product over a block.

  check_microscaling_legality(W, LMUL, EEW_C, lambda) — checks the
    SEW×λ≥16 and BS=16 LMUL constraints.

Each instruction body now consists of instruction-specific checks
(vm=0 reserved, SEW restrictions, altfmt checks) followed by a call
to decode_gemm_geometry and a compact main loop using the helpers.
Tile load/store instructions (vmtl, vmts, vmttl, vmtts) are unchanged.

Author: ptomsich