Adds shove_needed.dsn: one signal layer (no vias), where NET_A routes as
a straight wall that boxes NET_C out under greedy ordering. Tests assert
the contrast: without shove NET_C is dropped (result still DRC-clean, just
incomplete); with shove NET_A is moved aside and both nets connect, the
exact has_violation check is clean, endpoints stay on pads, and the result
is deterministic. Regression tests confirm shove is a no-op on the simple/
crossing/ripup/kicad boards (same connectivity, DRC-clean, zero shoves).
Oracle-gated parity is included (skips when the JAR cannot route the
synthetic single-layer board).
Ryan Malloy
e58f63e0f1