docs: clean up, split up, restructure README.md

docs/TRACING.md

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+# Tracing Support
+
+dagx provides optional observability through the `tracing` crate with **zero runtime overhead when disabled**. The tracing instrumentation is conditionally compiled using feature flags, meaning when the feature is off, the logging code doesn't exist in the compiled binary at all.
+
+## Enabling Tracing
+
+```toml
+[dependencies]
+dagx = { version = "0.3", features = ["tracing"] }
+tracing-subscriber = "0.3"
+```
+
+## Usage Example
+
+```rust
+use tracing_subscriber::{fmt, EnvFilter};
+
+// Initialize tracing subscriber
+fmt()
+    .with_env_filter(
+        EnvFilter::try_from_default_env()
+            .unwrap_or_else(|_| EnvFilter::new("dagx=info"))
+    )
+    .init();
+
+// Your DAG code here - tracing will log execution details
+```
+
+## Log Levels
+
+- **INFO**: DAG execution start/completion
+- **DEBUG**: Task additions, dependency wiring, layer computation
+- **TRACE**: Individual task execution (inline vs spawned), detailed execution flow
+- **ERROR**: Task panics, concurrent execution attempts
+
+Run with different levels:
+
+```bash
+RUST_LOG=dagx=info  cargo run    # High-level execution info
+RUST_LOG=dagx=debug cargo run    # Task and layer details
+RUST_LOG=dagx=trace cargo run    # All execution details
+```
+
+See [`examples/tracing_example.rs`](../examples/tracing_example.rs) for a complete working example.
+
+## Zero-Cost Guarantee
+
+When the `tracing` feature is disabled (the default), there is **literally 0ns overhead**:
+
+- Logging code is removed at compile time via `#[cfg(feature = "tracing")]`
+- No branches, no function calls, nothing
+- The `tracing` crate isn't even linked
+- Benchmarks verify identical performance with/without the feature
+
+This follows the same pattern used by Tokio, Hyper, and other performance-critical Rust async libraries.