riscv: DIV, DIVU, REM and REMU are neither constant-time nor DOIT

[clebreto]

Fixes #1013.

RISC-V has no run-time flag akin to Intel DOIT or ARM DIT; the reference for data operand independent timing is the ratified Zkt extension ("Data-Independent Execution Latency Subset", chapter Zkt of the Scalar Cryptography spec). Its safe list covers, for the RV32IM subset modelled by Jasmin, all the base ALU instructions and mul/mulh/mulhu/mulhsu, and deliberately excludes division and remainder.

Changes:

• riscv_instr_decl.v: the DOIT/NOT_DOIT constructors are now passed explicitly to the descriptor helpers at every call site (as done for the other architectures in arch: carry the DOIT classification in the instruction descriptors #1520); DIV, DIVU, REM and REMU become NOT_DOIT. A documentation block records the classification rationale against the Zkt list, including:
  • pseudo-instructions, classified by their canonical expansions (MV → addi, NOT → xori, NEG → sub, LI → lui+addi, LA → auipc+addi), all Zkt-listed;
  • loads and stores, which Zkt does not cover: the address is already leaked by the Jasmin leakage model, and independence from the transferred values is assumed, as on the other architectures (Intel's DOIT list contains MOV, ARM's DIT covers LDR/STR), so they stay DOIT;
  • the HINT caveat raised in the issue: Zkt exempts HINT encodings (typically rd = x0) from the latency requirement, but Jasmin can never emit one since x0 is not part of the declared register file.
• riscv_arch_full.ml: is_ct_asm_op returns false for DIV/DIVU/REM/REMU (the FIXME RISCV is resolved), matching the x86/ARM convention of assuming everything but division to be constant-time.
• Tests: new CCT-riscv and CCT-DOIT-riscv categories with six tests: secret #DIV/#REM rejected by the plain CT checker, secret / and % rejected under --doit (both were wrongly accepted before this change), public division accepted, and secret mul/add/xor/shift/spill accepted under --doit.
• The Zkt reference is added next to the Intel and ARM ones in docs/source/tools/ct.md.

Validation: full proofs build, extraction and compiler build are green; CCT, CCT-riscv, CCT-DOIT, CCT-DOIT-riscv, SCT (43/43) and the risc-v compilation category (106/106) all pass.