feat(sdk): Add tracing integration for structured logging

[lowhung]

Summary

Adds optional tracing integration for structured logging and debugging of SDK operations.

Changes

• Add tracing feature flag to buswatch-sdk
• Add structured logs at appropriate levels:
  • TRACE: record_read/record_write operations (with module, topic, count fields)
  • DEBUG: Module registration, snapshot collection
  • INFO: Background emission start/stop
  • WARN: Emission failures
• Zero overhead when feature is disabled (conditional compilation)

Usage

[dependencies]
buswatch-sdk = { version = "0.1", features = ["tracing"] }

use tracing_subscriber;

fn main() {
    tracing_subscriber::fmt::init();
    
    let instrumentor = buswatch_sdk::Instrumentor::new();
    let handle = instrumentor.register("my-service"); // DEBUG log
    handle.record_read("events", 10); // TRACE log
}

Control verbosity:

RUST_LOG=buswatch_sdk=debug cargo run

Testing

• All 42 SDK tests pass with tracing feature enabled
• All tests pass without feature (conditional compilation verified)

Closes #22