Harden packaging for a public release

Add the GPLv3 text (the package declared the licence but never shipped it),
a py.typed marker, and an sdist include/exclude allowlist so the distribution
carries only src/, docs/, README, LICENSE and pyproject. reference/ is a clone
of the GPL FreeRouting Java tree kept purely as a porting reference and must
never be redistributed inside this package; tests, caches and build output are
excluded too.

Switch to the PEP 639 SPDX licence expression with license-files, widen the
classifiers, and add an -o alias for -do. Rewrite the README around the engine
matrix: what each track and pass actually does, and what this is not (no
FreeRouting density parity on dense boards, and --diagonal is a recovery and
shortening pass, not a diagonal-native search).

src/freeroute/cli.py

@@ -70,7 +70,7 @@ def build_parser() -> argparse.ArgumentParser:
         "-de", "--design", dest="design", help="input .dsn (FreeRouting-compatible alias)"
     )
     parser.add_argument(
-        "-do", "--output", dest="output", help="output .ses file (default: stdout)"
+        "-do", "-o", "--output", dest="output", help="output .ses file (default: stdout)"
     )
     parser.add_argument(
         "-mp",