fft_engine_axi_bridge: respect axi_din_tready with 1-deep skid buffer

Bug: bridge advanced in_count and asserted tlast on din_valid alone,
ignoring the IP's tready handshake. With LogiCORE FFT v9.1 in
nonrealtime throttle mode (per .xci), tready can deassert briefly
during BFP normalization or pipeline events, silently dropping input
samples and shifting tlast off-by-N.

Fix: add 1-deep skid buffer + AXI-correct handshake. Phase 1 drains
the active beat when the IP accepts it (and shifts skid up); Phase 2
loads new upstream samples respecting post-handshake slot availability.
Track accept_count separately from in_count to drive the S_FEED->S_DRAIN
transition on the Nth accepted beat. Sustained 2+ cycle backpressure
exhausts the skid and sets overflow_sticky for debug visibility.

Audit cross-refs (AUDIT-C10):
- "tready ignored" - CONFIRMED, fixed here
- "SCALE_SCH unset" - REFUTED (BFP mode uses tuser, not cfg_tdata)
- "output ordering not configured" - REFUTED (.xci natural_order)

Verification: new tb_fft_engine_axi_bridge.v with stub xfft_2048
exercises 4 backpressure patterns (none / dip-at-3 / dip-at-100 /
3-cycle sustained). Quick regression 30/30 PASS.

9_Firmware/9_2_FPGA/run_regression.sh

@@ -546,6 +546,10 @@ run_test "FPGA Self-Test" \
     tb/tb_fpga_self_test.vvp \
     tb/tb_fpga_self_test.v fpga_self_test.v
 
+run_test "FFT AXI Bridge tready handshake (AUDIT-C10)" \
+    tb/tb_fft_engine_axi_bridge.vvp \
+    tb/tb_fft_engine_axi_bridge.v fft_engine_axi_bridge.v
+
 echo ""
 
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