test: certify runtime width early-termination (C.2), document BUG-003/004

n_inputs_real/n_neurons_real early termination for valid values is
certified real: a "poison" region (data that would saturate the result
if read past the claimed limit) confirms no over-read, cycle counts
scale proportionally. n_inputs_real non-multiple-of-PARALLEL at runtime
matches the documented silent-truncation risk exactly.

n_inputs_real=0 / n_neurons_real=0 (BUG-003/004): confirmed incorrect
behavior in every repetition, but the exact triggering mechanism was
NOT fully isolated -- nearly-identical repeated tests produced
different symptoms (clean hang vs. silently processing the full build
width vs. a third cycle count matching neither). Reported in full,
including the inconsistency itself, rather than picking the cleanest
result. The two new permanent testbenches reflect this honestly: the
solid early-termination checks are hard assertions, the n_*_real=0
probe is deliberately observe-only given the non-deterministic result.

Full regression: 38/38 real tests pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013xXuuRUWZScuo1DeYJxs3v

sim/layer.vcd

@@ -1,5 +1,5 @@
 $date
-	Fri Sep  4 13:40:17 2026
+	Fri Sep  4 14:26:01 2026
 $end
 $version
 	Icarus Verilog