fix(rtl): force FIR adder tree to fabric to free 30 DSPs for FFT butterfly on 50T

Add (* USE_DSP = "no" *) attribute to FIR lowpass adder tree registers
(add_l1, add_l2, add_l3, accumulator_reg) to prevent Vivado from
inferring DSP48E1 slices for pure addition operations.

Each fir_lowpass_parallel_enhanced instance was using 47 DSPs (32 for
multiply + 15 for the adder tree). The 15 adder-tree DSPs per instance
(30 total for I/Q pair) performed only PCIN+A:B additions with no
multiplier usage. On the XC7A50T with only 120 DSP48E1 slices, this
caused 100% DSP utilization and forced FFT butterfly complex multipliers
to spill into 18-level fabric carry chains (WNS=-1.103ns).

Moving these 36-bit additions to fabric CARRY4 chains (~9 CARRY4 per
add, ~2ns propagation) is well within the 10ns clock period and frees
~30 DSPs for the FFT engine to use native DSP48E1 multipliers.

Regression: 23/23 FPGA tests PASS (attribute is synthesis-only).

9_Firmware/9_2_FPGA/fir_lowpass.v

@@ -57,13 +57,16 @@ wire signed [DATA_WIDTH+COEFF_WIDTH-1:0] mult_result [0:TAPS-1];
 // Level 0: 16 pairwise sums of 32 products
 reg signed [ACCUM_WIDTH-1:0] add_l0 [0:15];
 // Level 1: 8 pairwise sums
-reg signed [ACCUM_WIDTH-1:0] add_l1 [0:7];
+// USE_DSP="no" forces pure additions to fabric CARRY4 chains, freeing DSP48E1
+// slices for the FFT butterfly multipliers that otherwise spill to 18-level
+// fabric carry chains causing timing violations on the XC7A50T (120 DSP budget).
+(* USE_DSP = "no" *) reg signed [ACCUM_WIDTH-1:0] add_l1 [0:7];
 // Level 2: 4 pairwise sums
-reg signed [ACCUM_WIDTH-1:0] add_l2 [0:3];
+(* USE_DSP = "no" *) reg signed [ACCUM_WIDTH-1:0] add_l2 [0:3];
 // Level 3: 2 pairwise sums
-reg signed [ACCUM_WIDTH-1:0] add_l3 [0:1];
+(* USE_DSP = "no" *) reg signed [ACCUM_WIDTH-1:0] add_l3 [0:1];
 // Level 4: final sum
-reg signed [ACCUM_WIDTH-1:0] accumulator_reg;
+(* USE_DSP = "no" *) reg signed [ACCUM_WIDTH-1:0] accumulator_reg;
 
 // Valid pipeline: 9-stage shift register (was 7 before BREG+MREG addition)
 // [0]=BREG done, [1]=MREG done, [2]=L0 done, [3]=L1 done, [4]=L2 done,