unprivileged/integrated-matrix: Address internal review feedback

Process all 28 items from the IME TG internal review feedback tracker.

Subextension dependencies (#3):
  Replace blanket Zve64d dependency with the minimum Zve subset per
  subextension: Zve32x for integer accumulators ≤ 32-bit, Zve64x for
  Int64 accumulators, Zve32f for FP accumulators ≤ 32-bit, and Zve64d
  only for FP64 accumulators.

8× widening instructions (#7, #8, #9, #24):
  Add v8wmmacc.vv (funct6=0x3b, OPIVV), vf8wmmacc.vv (funct6=0x17,
  OPFVV), and vf8wimmacc.vv (integer-input MX variant, vm=0 of
  v8wmmacc) with full instruction definitions, SAIL pseudocode,
  encoding diagrams, and exception tables.  Update encoding maps (FP,
  integer, integer MX) with W=8 entries.  Add Zvvxi4fp32mm and
  Zvvxni4fp32mm to the MX subextension table.  Replace the informative
  NOTE about reserved W=8 encoding space with normative text.  Remove
  the undefined term "octal-widening".

MXINT4 clarification and OCP citation (#14):
  Define MXINT4 as analogous to OCP MX's MXINT8 but with 4-bit signed
  elements.  Add proper citation of the OCP Microscaling Formats (MX)
  v1.0 Specification with URL.  Update microscaling applicability to
  include vf8wmmacc.vv.

vfmmacc.vv vm=0 cleanup (#13, #28):
  Remove contradictory "When vm=0" exception bullets (vm=0 is reserved
  for non-widening FP).  Replace dead microscaling SAIL code with a
  straightforward non-widening FP GEMM loop.  Add explicit note that
  microscaling is not supported for non-widening multiply-accumulate.

Terminology fixes (#15, #21):
  Add forward cross-reference at first use of altfmt_A/altfmt_B.
  Correct two occurrences where λ was described as "the K dimension"
  to "tile-layout parameter", clarifying that K_eff = λ × W × LMUL is
  the derived effective K dimension.

Author: ptomsich