feat(dec): Decaps D6 - c' = K-PKE.Encrypt(ek_pke, m', r')

Re-encryption step of the FO transform (FIPS 203 Alg 17 step 8), done by
reusing the ENTIRE Encaps E1-E7 pipeline rather than duplicating it:

- FSM: ST_DEC_J (D5) -> ST_ENC_LOAD, then the existing Encaps chain
  LOAD->A->C->N->U->C1->TDEC->E2MV->V->C2 runs unchanged and writes c' to
  ct_bram. The reuse preconditions are all in place: rho loads from ek_bram's
  ek_pke region (same 384k offset Encaps uses; populated at D0 load via
  dk_ld_ekpke), the CBD seed is r_r (r' from D5), and ek_pke is in ek_bram.
- D4 now packs the recovered message directly into m_r (dropping the separate
  mprime_r register): Encaps V's mu reads m_r[idx] and dbg_mprime_o now aliases
  m_r, so the re-encrypt sees m' with no extra plumbing.
- ST_ENC_LOAD arming generalized to fire when entered from ST_ENC_G (Encaps)
  or ST_DEC_J (Decaps re-encrypt).

The re-encrypt overwrites bank_a/bank_se/bank_t, so the bank-based stage checks
(D1 v', D2 s_hat/u_hat, D3 w) are no longer valid at end-of-run. The dec TB now
verifies the surviving register/BRAM artifacts: dk parse (D0), m' (D4, in m_r),
K'/r'/K-bar (D5), and the 768/1088/1568-byte c' against golden (D6). Earlier
stages remain proven by their per-stage builds and transitively by c'.

Verified: dec D6 K=2/3/4 all cases PASS (c' == golden == valid ciphertext c);
KeyGen + Encaps unregressed.

.claude/plans/decaps_plan.md

@@ -67,7 +67,7 @@
 - **D3 — w = v' − INTT(Σ s∘u_hat)** ✅:复用 Encaps V 机(ST_DEC_W,u_row=0 单多项式)。MAC s_hat[j](bank_a slot j*K)∘ u_hat[j](bank_se rel j)→psum bank_t[UPSUM],与 V MAC 完全同址,免改。INTT 原地。SUB:w = v' − psum,(v'−psum) 负则 +Q。**关键:v'/psum 读口冲突 → D1 把 v' 落到 bank_a slot DEC_VASLOT=1(s_hat 在 j*K,slot 1 恒空 K≥2),SUB 时 psum(bank_t)+v'(bank_a)并行读,正如 V-ADD 并读 psum+e2。** K=2/3/4 w 全过。
 - **D4 — m' = byteEncode₁(Compress₁(w))** ✅:ST_DEC_MENC,逐系数读 bank_t[UPSUM] 的 w,Compress₁(w)=1 iff 832<w≤2496(Q=3329),LSB-first 打包进 mprime_r[255:0],经 dbg_mprime_o 暴露。TB verify_d4 对 32 字节 golden(== KAT 解密的 m'==原始 m)。K=2/3/4 全过,KG/Enc 回归通过。K-PKE.Decrypt 硬件全链路打通。
 - **D5 — G(m'‖h) → (K',r') + J(z‖c) → K̄** ✅:G 复用单块 SHA3-512 路径(ST_DEC_G,mode=11,dec_g_data={hek_r,mprime_r}),输出 K'→ss_r(候选 ss,ss_o),r'→r_r(D6 PRF 种子,dbg_r_o)。J 复用多块吸收口(ST_DEC_J,mb_en=1),仿照 H(ek) 机器组装 136B/块,字节源 g<32 取 z_r、32≤g<msglen 取 c_in_bram、否则 SHAKE256 pad(0x1F 后缀,末字节|0x80,与 H 的 0x06 唯一区别)。mb_* 端口按 ST_DEC_J 在 H/J 间多路选择。K̄→kbar_r(dbg_kbar_o)。**踩坑:c_in_bram 读经 cin_rd_addr_r 寄存器 + BRAM 寄存 = 2 拍延迟,而组装/写回流水只容 1 拍 → z→c 边界首个 c 字节(byte32)读到 X,毒化整个 keccak。改为 ST_DEC_J 时 cin_rd_addr 组合直接取 dj_c_idx(去掉寄存器级),数据次拍到达正好对齐写回。** K=2/3/4(密文 768/1088/1568B → 6/9/12 块)全过;K'==KAT ss(有效密文)验证 FO 正确;KG/Enc 回归通过。
-- **D6 — c' = Encrypt(ek_pke,m',r')**:复用 Encaps E1–E7 写 ct_bram。dbg 对 c'==KAT.ct(有效 ct 时)。
+- **D6 — c' = Encrypt(ek_pke,m',r')** ✅:整条 Encaps E1–E7 流水直接复用——ST_DEC_J 完成后跳 ST_ENC_LOAD(rho 从 ek_bram 的 ek_pke 区载入,与 Encaps 同 offset 384k),经 A→C→N→U→C1→TDEC→E2MV→V→C2 跑完写 ct_bram。复用前提全部就位:r'(CBD 种子)在 r_r、ek_pke 在 ek_bram(D0 load 时 dk_ld_ekpke 写入)。**关键改动:D4 把 m' 直接打包进 m_r(而非独立 mprime_r),因为 Encaps V 的 mu=m_r[idx]、dbg_mprime_o 也改指 m_r;省一个寄存器且让重加密天然读到 m'。** ST_ENC_LOAD 的 arming 扩展为从 ST_ENC_G 或 ST_DEC_J 进入皆触发。**注意:重加密覆盖 bank_a/se/t,故 D1–D3 的 bank 检查在 run 末已失效;TB 改为只校验存活的寄存器/BRAM 工件(D0 解析、m'、K'/r'/K̄、c'),bank 阶段正确性由早期分阶段构建 + c' 传递性保证。** K=2/3/4 c'==golden(==有效密文 c)全过,KG/Enc 回归通过。
 - **D7 — 比较 + 拒绝 mux + 端到端 KAT**:c'==c 常量时间比较,ss=mux。干净 TB:
   - 有效 ct(KAT.ct):ss==KAT.ss(c'==c → K')。
   - 损坏 ct(KAT ct_n / ss_n):ss==KAT.ss_n(c'≠c → K̄)。