feat(routing): M1.2.6 Message Routing Middleware (#205)

[LyeZinho]

Summary

• Implement M1.2.6: Message routing middleware with MQTT-style topic-based dispatch
• Key features: Topic/pattern validation, wildcard matching (* and #), thread-safe handler registration
• Test coverage: 19 unit tests + 5 integration tests (37/37 pass)
• Code quality: fmt ✓, clippy ✓, build ✓

Commits

• feat(routing): Define RoutingError enum with Display trait
• docs: Add M1.2.6 message routing middleware design document
• docs: Add M1.2.6 message routing implementation plan
• feat(routing): Add Topic, TopicPattern, MessageRouter with wildcard matching and 18 tests
• test(integration): Add comprehensive routing integration tests with threading and hierarchy
• fix(routing): Use or_default() instead of or_insert_with(Vec::new) per clippy

Design

Message routing implements MQTT-style topic hierarchies with pattern matching:

• Topics: mimi/commands/*, mimi/events/*, mimi/memory/*, etc.
• Patterns: exact, * (single level), # (multi-level end-only)
• Thread-safe: Arc<Mutex<HashMap<String, Vec>>>
• Fan-out: Failed handlers logged but don't block other handlers

Async handlers documented as M1.3+ future work.