feat(sha3): multi-block SHA3-256 absorb for H(ek); KeyGen golden vectors

Stage 0+1 of mlkem_top KeyGen integration:
- sha3_top: add multi-block SHA3-256 absorb FSM (mb_en/mb_block_i/mb_valid_i/
  mb_last_i/mb_ready_o). Caller pre-pads final block; module does pure absorb
  loop (state^=block; Keccak-p). Single-block G/H/J paths bit-identical when
  mb_en=0. Sticky digest register holds output until consumer acks.
- tb_sha3_mb_xsim: self-checking TB streams 800B ek (6 blocks) -> H(ek),
  verified == hashlib.sha3_256. Proper valid/ready handshake (no force).
- Existing G/H/J TBs (xsim + Verilator) tie off mb_* ports; both frameworks
  regress clean (Verilator 25/25, XSIM G/H/J + keccak + 7-vec + multiblock).
- test_framework/modules/mlkem_keygen/golden: full 256-coeff per-stage
  intermediates (rho/sigma, A_hat, s/e, s_hat/e_hat, t_hat, ek, dk_pke) for
  KAT count=0..4, dumped by ml-kem-r and self-verified against NIST KAT.

test_framework/modules/mlkem_keygen/golden/c000_dkpke.hex

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