tor: serialize lifecycle transitions (ENV-3072)

[Jacksper13]

Problem

Tor lifecycle calls can overlap. A stop issued while start is bootstrapping can return before that stale start publishes a live SOCKS proxy, and bootstrap can similarly restore readiness after teardown.

Changes

• serialize start, bootstrap, and stop operations
• invalidate in-flight generations as soon as stop is requested
• tear down resources returned by a superseded start instead of publishing them
• stop publishing the route before native teardown begins
• add deterministic ordering and invalidation regression tests

Stack

This is stacked on #81 so it builds on the final listener ownership and deterministic teardown introduced by #79 and #81. Only this PR's lifecycle changes are shown against that base.

Validation

• flutter test test/tor_lifecycle_gate_test.dart
• flutter analyze --no-pub lib/tor.dart lib/src/tor_lifecycle_gate.dart test/tor_lifecycle_gate_test.dart
• dart format
• git diff --check

Tracks ENV-3072.