feat: base subscribers on tracing

[willkill07]

Overview

Move NeMo Relay subscriber/event delivery toward Rust tracing and tracing-subscriber while preserving the existing named subscriber APIs across bindings and adding closeable subscription handles.

• I confirm this contribution is my own work, or I have the right to submit it under this project's license.
• I searched existing issues and open pull requests, and this does not duplicate existing work.

Details

• Adds core tracing emission for canonical ATOF lifecycle events and a callback-backed Subscriber that implements both tracing::Subscriber and tracing_subscriber::Layer.
• Adds anonymous subscription handle APIs in Rust and additive handle APIs for Python, Node.js, Go, WebAssembly, and FFI while keeping named APIs unchanged.
• Preserves canonical Event/ATOF delivery for existing subscribers and exporters, including ATIF, ATOF JSONL, OpenTelemetry, and OpenInference.
• Adds scope-local subscription cleanup semantics, FFI close/free coverage, binding tests, and docs for the tracing/subscriber model.

Validation run locally:

• cargo fmt --all
• go fmt ./...
• cargo clippy --workspace --all-targets -- -D warnings
• just test-rust
• just test-python
• just test-node
• just test-go
• just test-wasm
• just docs
• git diff --check

Where should the reviewer start?

Start with crates/core/src/api/runtime/state.rs for tracing-first event emission and subscriber snapshot caching, then crates/core/src/api/subscriber.rs for the Rust subscriber/layer and handle APIs. The cross-binding API additions are covered by crates/ffi/src/api/subscription.rs, crates/python/src/py_api/mod.rs, crates/node/src/api/mod.rs, crates/wasm/src/api/mod.rs, and go/nemo_relay/nemo_relay.go.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Relates to: none

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