feat: process-reuse support + native log redirection (1.3.0)

[FeodorFitsner]

Summary

• New C exports + Python module API for handling Android process reuse — the scenario where the OS keeps the OS process alive after a back-button quit and restarts only the Dart VM. The old socket-based plugin coped via "skip Py_Initialize if Python is already up" + a still-running socket Flet server. The current dart_bridge transport had neither, so process-reuse would either crash (re-init) or silently hang (stale port handlers).
• sys.stdout / sys.stderr redirection to native log sinks (logcat / os_log / stderr) installed right after Py_Initialize — restores the old plugin's adb logcat -s flet.python:* visibility lost during the dart_bridge migration.

Wire / API contract

New C exports (callable from Dart via FFI):

int  dart_bridge_is_python_initialized(void);
void dart_bridge_signal_dart_session(int n_pairs,
                                      const char* const* labels,
                                      const int64_t* ports);
int  dart_bridge_install_stdio_redirect(void);

New Python module method (built-in dart_bridge):

dart_bridge.add_session_restart_handler(callback)
# callback receives a {label: port} dict on every new Dart VM signal.

Behavioral change in serious_python_run(): when called with Py_IsInitialized() already true, no longer refuses with an error — instead applies the new env vars and fires dart_bridge_signal_dart_session with the labeled port map reconstructed from this run's config ("protocol" ← FLET_DART_BRIDGE_PORT, "exit" ← FLET_DART_BRIDGE_EXIT_PORT). Returns 0 immediately.

Registered Python handlers (flet's FletDartBridgeServer restart loop, the flet build template's sys.exit patcher) rewire to the new ports — user-level Python state and singletons are preserved across the Dart VM restart.

Native log redirection

Right after Py_Initialize, sys.stdout / sys.stderr are replaced with file-like wrappers whose write() forwards to a platform-specific native log sink:

• Android: __android_log_write under tag flet.python (INFO for stdout, ERROR for stderr — visible via adb logcat -s flet.python:*)
• iOS/macOS: os_log_with_type (visible in Console.app)
• Other (desktop Linux/Windows): fwrite passthrough — preserves flet run console output and CI logs unchanged

Installed by dart_bridge_install_stdio_redirect(), called from sp_run_python() immediately after Py_Initialize so even bootstrap-time prints (sys.path injection, top-level user-app statements) land in the platform log instead of vanishing into /dev/null on mobile.

Implementation is at the Python sys.stdout layer (not fd 1/2 replacement via dup2 + pipe + reader thread) — simpler, no extra thread, catches print() and traceback writes directly. Native crashes that bypass Python's stdio aren't captured here (separate concern).

Other changes

• dart_bridge_clear_handlers also clears the session-restart subscriber list, since those PyObject* refs are held the same way as port handlers.
• CMakeLists.txt: version bump 1.2.2 → 1.3.0 (binary-compatible adds — existing consumers keep working without code changes; the new exports are looked up softly on the Dart side via lookupOrNull).

Test plan

• Local desktop build (macOS): cmake --build clean, all four new exports present in nm -gU libdart_bridge.dylib.
• ctypes smoke test of dart_bridge_is_python_initialized + dart_bridge_signal_dart_session against the "no embedded interpreter" code path — no crashes.
• CI matrix build (Linux / Windows / Android / Apple xcframework).
• End-to-end Android process-reuse test: launch app → back-button exit → re-tap icon (OS reuses process) → confirm UI re-appears with fresh session, no Py_Initialize crash in logcat, flet.python tag carries app's print output.
• Cross-platform fresh-start regression: flet build apk / macos / windows / linux builds — is_python_initialized returns 0 on first run, signal_dart_session is a no-op, behavior unchanged from 1.2.2.

Consumer rollout

• flet-dev/serious-python: Dart FFI bindings for the new exports use lookupOrNull so pre-1.3.0 binaries still load (calls degrade to no-op). Companion branch ready.
• flet-dev/flet: flet.app.run_async dart_bridge branch grows a _DartBridgeServerHandle wrapper that swaps in a new FletDartBridgeServer on add_session_restart_handler callback. flet build template's initBridges calls signalDartSession on every startup. python.dart bootstrap makes _exit_port mutable and subscribes to restart events.

Plan doc: the-new-challenge-at-joyful-thacker.md (local plan file).