feat(dec): Decaps D0 - op_i widen + dk/c load + parse

Scaffolding for ML-KEM Decaps (FIPS 203 Alg 18):
- op_i widened to 2-bit: 00=KeyGen, 01=Encaps, 10=Decaps (op_r too).
- New ST_DEC_LOAD state (D0: settles to DONE so load/parse is dbg-checkable).
- dk (=sk) streamed via dk_in_*; load logic routes each byte by region:
  [0,384K)->dk_pke (dkp_bram), [384K,768K+32)->ek_pke (ek_bram),
  [768K+32,+32)->H(ek) (hek_r), [768K+64,+32)->z (z_r). Routing uses the
  LIVE k_i input, not start-captured k_r (dk is streamed before start_i).
- c (=ct) streamed via c_in_* into a SEPARATE c_in_bram, so the computed c'
  (ct_bram) can later be compared against original c and J(z||c) can read c.
- New dbg taps: dbg_mprime_o/dbg_kbar_o/dbg_decz_o/dbg_dech_o.

TB: tb_mlkem_dec_katK_xsim verifies dk parse (H(ek), z, ek_pke/dk_pke BRAM
round-trip). gen_decaps_vectors.py emits dec_k{K}_c{N}_{dk,ct,ss,ctn,ssn}.hex
from the NIST KAT. run_tb.sh gains a 'dec' module (mirrors 'enc').

Regression fix: old KeyGen/Encaps TBs didn't connect the new input ports,
floating them to X and corrupting the ek/dkp write muxes -> tied off
dk_in_*/c_in_*/new dbg taps in both.

Verified: dec D0 K=2/3/4 PASS; KeyGen K=2 + Encaps K=2 unregressed.

sync_rtl/top/TB/tb_mlkem_enc_katK_xsim.v

@@ -27,16 +27,19 @@ module tb_mlkem_enc_katK_xsim;
     wire [255:0] dbg_rho_o, dbg_sigma_o, dbg_r_o, dbg_hek_o;
 
     mlkem_top dut (
-        .clk(clk), .rst_n(rst_n), .k_i(k_i), .op_i(1'b1),
+        .clk(clk), .rst_n(rst_n), .k_i(k_i), .op_i(2'd1),
         .d_i(d_i), .z_i(z_i), .msg_i(m_i), .start_i(start_i),
         .busy_o(busy_o), .done_o(done_o),
         .ek_in_we(ek_in_we), .ek_in_addr(ek_in_addr), .ek_in_byte(ek_in_byte),
+        .dk_in_we(1'b0), .dk_in_addr(12'd0), .dk_in_byte(8'd0),
+        .c_in_we(1'b0), .c_in_addr(11'd0), .c_in_byte(8'd0),
         .ss_o(ss_o), .dbg_ct_idx_i(dbg_ct_idx_i), .dbg_ct_o(dbg_ct_o),
         .dbg_slot_i(dbg_slot_i), .dbg_idx_i(dbg_idx_i), .dbg_coeff_o(dbg_coeff_o),
         .dbg_byte_sel_i(dbg_byte_sel_i), .dbg_byte_idx_i(dbg_byte_idx_i), .dbg_byte_o(dbg_byte_o),
         .dbg_dk_idx_i(dbg_dk_idx_i), .dbg_dk_o(dbg_dk_o),
         .dbg_rho_o(dbg_rho_o), .dbg_sigma_o(dbg_sigma_o),
-        .dbg_r_o(dbg_r_o), .dbg_hek_o(dbg_hek_o)
+        .dbg_r_o(dbg_r_o), .dbg_hek_o(dbg_hek_o),
+        .dbg_mprime_o(), .dbg_kbar_o(), .dbg_decz_o(), .dbg_dech_o()
     );
     always #5 clk = ~clk;