test: certify graph_engine gather/guard (C.6), find related BUG-006

Gather/padding/src_id<out_id guard certified via existing solid
pre-session tests (graph_engine_tb.v checks act_buffer contents via
hierarchical reference, not just final output; graph_engine_guard_tb.v
covers 4 adversarial cases incl. recovery).

BUG-006 (LOW severity): num_neurons_graph=0 shares BUG-005's exact
root cause (neuron_idx is a full 16-bit register, no guard), but
graph_engine's existing per-edge src_id<out_id guard incidentally
catches most garbage-data patterns fast (err at cycle 58 for a
non-trivial test pattern, vs. layer_sequencer's 21761-cycle full run
in BUG-005) -- not a designed protection for this case, so not closed
as a non-issue, but lower severity given the observed practical risk.
Not run to full 65536-iteration completion (impractical for this
campaign's time budget) -- limitation stated explicitly.

Full regression: 40/40 real tests pass, 1 new observational test.

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 14:46:23 2026
+	Fri Sep  4 14:51:45 2026
 $end
 $version
 	Icarus Verilog