Extend rust-development with advanced async patterns (Tower, CancellationToken, graceful shutdown)

[AlexMikhalev]

Parent Epic: #62

What

Add advanced async programming patterns to rust-development SKILL.md that are currently missing:

Sections to Add

1. Graceful Shutdown -- CancellationToken pattern with tokio, signal handling
2. Backpressure -- Bounded channels, vectored I/O batching
3. Tower Middleware Composition -- Service trait, Layer stacking, rate limiting, concurrency control, load shedding
4. Custom Futures -- Retry logic with exponential backoff as a Future implementation
5. Structured Concurrency -- Task lifecycle management, JoinSet patterns

Source Reference

Sections 3 (Async Programming with Tokio 1.44+) and 9 (Advanced Async Patterns) from RUST_SYSTEM_PROGRAMMING_BEST_PRACTICES.md

Disciplined Engineering Alignment

These patterns integrate with the disciplined-* skill phases as follows:

Phase	Skill	Rust Async Relevance
Research	disciplined-research	Identify async boundaries, cancellation requirements, backpressure needs in system under analysis
Design	disciplined-design	Specify Tower middleware stack, shutdown strategy, channel bounds in design doc
Implementation	disciplined-implementation	Apply CancellationToken/JoinSet patterns; each middleware layer = one reviewable step
Verification	disciplined-verification	Unit test cancellation paths, verify backpressure under load, test middleware ordering
Validation	disciplined-validation	Validate graceful shutdown behaviour in production-like environment; UAT for timeout/retry UX

The SKILL.md should include a "Disciplined Workflow Integration" subsection noting:

• During research: document async boundaries and failure modes
• During design: specify Tower layer stack and channel capacities
• During verification: test cancellation safety and backpressure behaviour

Acceptance Criteria

• SKILL.md updated with new async section(s) including code examples
• Existing async patterns section preserved (extend, don't replace)
• Crate recommendations table updated (tower, tower-http)
• Disciplined workflow integration notes included