fix(sample_ntt): suppress spurious 257th valid_o after last_o

Phase-1 (d1 output) lacked the 'need_more' guard that phase-2 (d2) had, so
when the 256th accepted coefficient was a d1 whose group then advanced, the
FSM could fire one extra valid_o after last_o for certain seeds (e.g. KAT
count=0 rho, seed i=0/j=1 emitted 257 pulses). In mlkem_top KeyGen this
trailing pulse leaked into the next poly's index 0, shifting the stream.

Fix: gate phase-1 d1 output with 'd1_acc_r && need_more' (mirrors phase-2).
Applied to both sample_ntt_sync and sample_ntt_sync_shared.

Standalone TB had a coverage blind spot (stopped reading at 256, never
checked valid_o stayed low after last_o). Added a regression assertion:
counts spurious post-last_o pulses and fails if any. Verified the assertion
catches the bug on a reverted-fix copy (3 spurious) and passes on the fix.

Verified:
- 40-seed audit (sync) + 24-seed audit (shared): all exactly 256 pulses,
  last_o@256, zero post-last pulses.
- Verilator vs hashlib oracle: 1536/1536 (no real coeff dropped).
- Full framework regression: 4334/4334.
- mlkem_top KeyGen Stage 2c: 2048/2048 A_hat+s+e coeffs exact.

sync_rtl/sample_ntt/sample_ntt_sync.v

@@ -334,7 +334,7 @@ module sample_ntt_sync #(parameter K = 4) (
 
                     // ----- Phase 1: output d1 -----
                     2'd1: begin
-                        if (d1_acc_r) begin
+                        if (d1_acc_r && need_more) begin
                             if (!valid_o_r) begin
                                 // First cycle: assert output
                                 coeff_o_r <= d1_r;
@@ -349,7 +349,7 @@ module sample_ntt_sync #(parameter K = 4) (
                                 end
                             end
                         end else begin
-                            // d1 rejected, skip to phase 2
+                            // d1 rejected or no longer needed: skip to phase 2
                             valid_o_r  <= 1'b0;
                             sq_phase_r <= 2'd2;
                         end