feat: add bounded Relay ATOF snapshots

[bbednarski9]

What

Summary

Adds the second independently functional NeMo Relay layer on top of #8:

• a router-neutral RelaySnapshotAccumulator in switchyard-components-v2::features;
• atomic finite-NDJSON ingestion at POST /v1/atof/events;
• exact (session_id, owner_id) snapshots with Relay-compatible identity fallbacks;
• deterministic identity, history, dedupe, request-size, and 64 MiB total retained-state bounds;
• optional bearer authentication through the Rust CLI, PyO3 binding, Python wrapper, and Python CLI;
• a checked-in Relay http_post / field_name_policy = "preserve" fixture.

This is the ATOF subscription and accumulation slice of a previous end-to-end implementation, split into a focused public-repository change.

Why

History-aware routers need one bounded, reusable snapshot source rather than per-router event state. This PR deliberately stops at ingestion and router-neutral reconstruction: Cascade projection and decision behavior remain in PR 3.

How tested

Testing summary

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --exclude switchyard-py
• cargo check -p switchyard-py
• uv run ruff check on all changed Python/test files
• uv run mypy switchyard/cli/switchyard_cli.py switchyard_rust/core.py switchyard_rust/server.py
• uv run maturin develop
• uv run pytest tests/test_serve_profile_config.py tests/test_route_bundle.py tests/test_profile_migration.py -q -o addopts= (97 passed)
• One-record Relay-style POST, multiline atomic rejection, auth, media type, event size, batch size, count pruning, total-byte pruning/accounting, global dedupe, canonical-event validation, and every identity fallback are covered.

Checklist

• One class per file; filename = snake_case of the primary class. (N/A: Rust/Python functional API change.)
• New public symbols exported from switchyard/__init__.py.__all__ if intended for downstream use. (N/A: Rust symbols are exported from the crate root; Python constants are exported from switchyard_rust.server.)
• Unit tests added for new components / bug fixes.
• README / --help updated if customer-facing surface changed.
• Commits signed off (Signed-off-by: Your Name <email>) per the DCO.

Notes for reviewers

Dependency and delta

This PR depends on #8. Because the author has read-only access to the upstream repository, this draft must target upstream main and temporarily contains the #8 commit as its prerequisite.

Review the #9 delta commits edd342d0 (snapshot memory/reporting fixes), 3857a695 (envelope-validation coverage), and e81cd9ec (shared Relay auth documentation), or use the exact fork delta. The earlier commits are prerequisites from #8 and the original ATOF implementation; after #8 merges, this branch will be rebased onto updated main.

Review guidelines

Suggested review order:

1. features.rs for atomic preparation, canonical event validation, identity resolution, global dedupe, and retained-byte accounting.
2. switchyard-server for optional auth, bounded body parsing, and the single server-owned accumulator.
3. Rust/Python CLI and binding propagation for every limit and the bearer token.
4. examples/relay_atof_switchyard.toml and CLI docs for the supported Relay transport contract.

The endpoint intentionally supports finite Relay transport = "http_post" requests (one event per POST or a finite multiline batch). Relay's long-lived transport = "ndjson" upload is out of scope because atomic batch application waits for request completion.

Previous feedback addressed

• Replaced the Cascade-specific accumulator with a reusable router-neutral snapshot contract while honoring the selected source location in components-v2::features.
• Bounded identity count, per-identity history, global dedupe entries, event bytes, batch bytes, and total retained-memory accounting bytes (64 MiB default).
• Parses and semantically validates the entire batch before taking the mutation lock; malformed recognized scopes cannot partially warm state.
• Restored global UUID/phase idempotency so changed identity metadata cannot bypass dedupe.
• Preserved the previous implementation's full compatibility fallback order for four session and seven owner sources, with precedence tests.
• Threads optional bearer auth through both Rust and Python startup paths; explicitly blank tokens fail configuration instead of silently disabling auth.
• Removes fallible test .expect() usage and documents the auth/body-validation contract.
• The Relay fixture pins http_post, field_name_policy = "preserve", and a replaceable bearer header without assuming unsupported header environment interpolation.

Relay-aware Cascade consumption, typed freshness, and unified request session metadata are intentionally deferred to #11.

Follow-up fixes

• Uses a read/write lock so concurrent snapshot readers do not serialize behind one global mutex.
• Makes the retained-state cap a conservative heap-accounting budget (serialized payload plus value/container overhead), rather than a wire-byte-only limit.
• Defines accepted_events as state-mutating ingests and exposes parsed received_events separately.