feat(ntt): implement synchronous NTT core with Barrett modular reduction

Phase 2.1: Merged Path00+Path01 NTT engine.
- barrett_mul.v: Barrett modular multiplication (a·b mod 3329)
- butterfly_unit.v: Cooley-Tukey/Gentleman-Sande butterfly
- zeta_rom.v: 128-entry ROM with bit-reversed roots of unity
- ntt_core.v: 7-layer NTT FSM, 256×12-bit register file
- ntt_sync.v: valid/ready streaming wrapper

Verified: 13/13 vectors bit-exact vs Python NTT/NTTInverse

sync_rtl/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