feat: rv64 mul mulh divrem

[876pol]

Resolves INT-6319.

Summary:

• mul, mulh, and divrem submodules of riscv have been ported from RV32 to RV64
  • All types/structs renamed from Rv32* to Rv64* (e.g., Rv32MultAdapterAir → Rv64MultAdapterAir, Rv32DivRemExecutor → Rv64DivRemExecutor)
  • Operand width changed from u32/4 bytes to u64/8 bytes throughout (registers, multiply/divide operations)
• Rv64MultAdapter (shared by all three submodules) updated for RV64
• Rv64M extension fully wired up in extension/mod.rs — executor enum, AIR registration, and chip creation all uncommented and updated (previously empty with impl_empty_executor! stub)
• Uncommented mul, mulh, and divrem in lib.rs