Move + rewrite bridge_example for PythonBridge API

bridge_example used to live alongside the serious_python_bridge plugin
(itself slated for deletion in Phase 2). With the v1.2.0 keyed-handlers
model and the new PythonBridge class in serious_python/lib/, the example
no longer needs serious_python_bridge — it depends only on serious_python.

Changes:
- git mv src/serious_python_bridge/example → src/serious_python/example/bridge_example
- pubspec drops serious_python_bridge; depends only on ../..
- lib/main.dart uses PythonBridge() / bridge.port / bridge.close()
- app/src/main.py reads BRIDGE_EXAMPLE_PORT env var and calls the new
  2-arg set_enqueue_handler_func(port, handler)
- integration_test exercises the same round-trip without the 8-byte
  handshake dance — Python knows its port from the env var the moment
  the interpreter starts
- README rewritten to reflect new architecture
- pubspec.lock gitignored (regenerates on path-deps anyway)

test-bridge-build.yml stripped to just the darwin example jobs pointed
at the new path. The obsolete cibuildwheel + NDK + linux/windows/android
example jobs will return as each platform plugin gets ported to download
pre-built libdart_bridge binaries.

Author: FeodorFitsner

.github/workflows/test-bridge-build.yml

@@ -47,3 +47,4 @@ app.*.map.json
 # serious_python packaging artifacts (built by `dart run serious_python:main package`)
 /app/app.zip
 /app/app.zip.hash
+pubspec.lock