EIP-7823: Set upper bounds for MODEXP --- MMU side

[OlivierBBB]

Note

Increase MODEXP micro-row totals to 64 and use the EIP-7823 upper byte-size bound in MODEXP_DATA preprocessing, updating TOT/TOTNT handling.

• MMU Constants:
  • Increase NB_MICRO_ROWS_TOT_MODEXP_ZERO and NB_MICRO_ROWS_TOT_MODEXP_DATA from 32 to 64 in mmu/osaka/constants.lisp.
• MODEXP_DATA preprocessing (mmu/osaka/instructions/modexp_data.lisp):
  • Compute left padding with EIP_7823_MODEXP_UPPER_BYTE_SIZE_BOUND instead of hardcoded 512.
  • Use NB_MICRO_ROWS_TOT_MODEXP_DATA when computing TOTNT (replaces hardcoded value) and rename the TOT-setting constraint for clarity.
• MODEXP_ZERO (mmu/osaka/instructions/modexp_zero.lisp):
  • Keep TOTNT tied to NB_MICRO_ROWS_TOT_MODEXP_ZERO (no logic change).

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