feat: add ATOF JSONL exporter

[willkill07]

Overview

Adds a native filesystem-backed ATOF JSONL exporter that registers as a NeMo Flow event subscriber and writes one raw ATOF Event JSON object per line.

• 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 nemo_flow::observability::atof::{AtofExporter, AtofExporterConfig, AtofExporterMode} with append/overwrite modes, default current-directory output, generated UTC filenames, registration, deregistration, flush, and shutdown.
• Exposes native bindings through FFI, Python, Node.js, and Go with language-appropriate config and lifecycle APIs.
• Keeps WebAssembly unsupported for this filesystem-backed exporter and documents the native-only limitation.
• Adds core, FFI, Python, Node.js, Go, and WASM coverage for defaults, lifecycle behavior, JSONL output shape, append/overwrite behavior, and unsupported WASM surface.
• Updates observability export docs with Rust, Python, Node.js, and Go usage examples.

Where should the reviewer start?

Start with crates/core/src/observability/atof.rs for the exporter behavior, then review crates/core/tests/unit/observability/atof_tests.rs and the FFI/Python/Node/Go binding additions for API parity.

Validation run locally:

• cargo fmt --all
• git diff --check
• just test-rust
• just test-python
• just test-node
• just test-go
• just test-wasm
• just docs
• cargo test -p nemo-flow observability::atof::tests -- --nocapture
• Commit hooks passed with lychee skipped after it reported two unrelated generated Rust API doc links under docs/reference/api/rust/_generated/nemo-flow-adaptive/...; just docs passed.

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

• Relates to: none

[willkill07]

/merge