Ryan Malloy 18f3afc8a5 Add FreeRouting JAR oracle harness for behaviour-level validation
FreeRouting ships no unit tests for its geometry/router, so there is no
value-level oracle to port against. This adds a dev/test-only harness that runs
the reference JAR to route a DSN, letting freeroute's output be diffed against
the reference implementation — the router phase will assert connectivity parity
(same nets routed) via routed_net_set().

- tests/oracle.py: locate Java 21+ and a freerouting JAR (env overrides:
  FREEROUTE_ORACLE_JAVA, FREEROUTING_JAR), route a DSN, and extract routed
  connectivity from the SES. requires_oracle skips when no JVM/JAR is present,
  so the suite stays Java-free.
- tests/test_oracle.py: routes a routable board end-to-end and checks the
  connectivity extraction.
- tests/dsn/fixtures/kicad_routable.dsn: a real KiCad pcbnew-exported DSN
  (Arduino_Mega template, path sanitized) that actually routes — the smd_demo
  fixture leaves its nets unrouted.
- pyproject: pythonpath=["tests"] so the harness imports as `oracle`.
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