feat: add ML-KEM accelerator with AXI slave wrapper

Wrap mlkem_top in an AXI4 burst slave (mlkem_axi_slave) mapped to the
freed FFT slot 6 at 0x1f400000 (virtual 0xbf400000). The core runs on
sys_clk/2 (25 MHz); the wrapper handles the clock-domain crossing:
start toggle->pulse, wait-stated byte-preload handshake, busy/done
sync, and direct readback taps (ek/dk/ct/ss). CPU drives it via a
register map (CTRL/STATUS/CONFIG, d/z/msg, preload + readback windows).

Wire into soc_top on the existing axiOut_6 nets (no crossbar change),
add mlkem_intr on intrpt[3]. Add sdk mlkem_test example running a
KeyGen->Encaps->Decaps self-consistency round trip.

rtl/ip/mlkem/ntt/ntt_sync.v

@@ -0,0 +1,19 @@
+module ntt_sync (
+    input         clk, rst_n,
+    input  [11:0] coeff_in,
+    input         valid_i,
+    output        ready_o,
+    output [11:0] coeff_out,
+    output        valid_o,
+    input         ready_i,
+    input         mode,
+    output        done_o
+);
+    ntt_core u_ntt_core (
+        .clk(clk), .rst_n(rst_n),
+        .coeff_in(coeff_in), .valid_i(valid_i), .ready_o(ready_o),
+        .mode(mode),
+        .coeff_out(coeff_out), .valid_o(valid_o), .ready_i(ready_i),
+        .done_o(done_o)
+    );
+endmodule