tools/fpga_benchmark.py: parametric Yosys + nextpnr-ecp5 benchmark
harness for the LFE5U-45F-8BG381 (speed grade -8, 80 MHz target),
sweeping PARALLEL over the neuron layer and parsing Fmax/LUT4/DFF/DSP
utilization out of the nextpnr report into JSON/CSV.
synth/ecp5/p2, p4, p8: real synthesis+PnR results backing the
same-price-tier FPGA comparison (P2: 87.88 MHz PASS, P4: 75.01 MHz
FAIL, P8: 147.62 MHz PASS -- non-monotonic, dominated by placement
noise since the whole design uses <2% of the device's LUT4 fabric at
every setting, and P8 notably maps to 0 DSP blocks vs 8/16 for P2/P4).
synth/ecp5/top.v: benchmark harness top-level, reworked to generate
deterministic non-constant X/weights/bias via `keep`-attributed
generate blocks so Yosys can't constant-fold the datapath away.
Also adds .gitignore for Python's __pycache__/*.pyc.
