feat(lib): AtomicNode runtime boundary (slice 1)

[joepio]

What

First slice of the node runtime boundary accepted in planning/runtime-boundary-decision.md: atomic_lib::runtime::AtomicNode, a thin struct wrapping Db that gives adapters one surface instead of each wrapping Db themselves.

No behaviour change. Every method delegates to a function that already existed:

AtomicNode	Delegates to
open(NodeConfig)	Db::init_memory / init_redb / init_redb_file / init_redb_opfs
get(subject, for_agent)	Storelike::get_resource_extended
query(&Query)	Storelike::query
apply_commit(json, IngestPolicy::Hub{..} | Peer)	sync::engine::ingest_commit (body extracted from ingest_commit_json, which now wraps it)
apply_commit(json, IngestPolicy::Replica)	sync::ws_apply::apply_commit_json (now returns CommitResponse; its one caller ignored the value)
apply_commit(json, IngestPolicy::LocalCache)	the former WASM applyCommit body (parse_json_ad_resource + no-validation CommitOpts)
mutate(ResourceEdit::Update | Genesis)	Resource::save_locally / save_as_genesis
subscribe()	Db::subscribe_events (existing DbEvent broadcast; no new bus)
sync_with_peer (iroh feature)	sync::peer::sync_drive_with_peer_outcome

IngestPolicy::{Hub, Peer, Replica, LocalCache} is named to match the CommitIngestOpts::{hub, peer, replica} presets in #1274 plus the WASM "fourth policy", so that PR can map onto it 1:1 once merged. Until then the enum carries literal CommitIngestOpts matching the server handler and apply_peer_commit.

First consumer: wasm/src/lib.rs ClientDb now holds an AtomicNode and routes applyCommit and query through it (getResource deliberately stays on the raw Db::get_resource since it has no for_agent).

Tests

lib/src/runtime/node.rs (--features db-redb):

• two AtomicNodes in one process: mutate(Genesis) on Alice, apply_commit(Peer) of the wire commit on Bob, get + query(parent) reflect it, and Bob's DbEvent::Changed stream includes the subject
• LocalCache accepts a commit with a tampered signature, Peer rejects it
• mutate without a default agent errors

Verified: cargo test -p atomic_lib --features db-redb --lib (429 passed), cargo clippy -p atomic_lib --features db-redb,iroh --lib --tests (no new warnings), cargo check -p atomic-wasm --target wasm32-unknown-unknown, cargo check -p atomic-server, cargo check --manifest-path flutter/rust/Cargo.toml.

Not in this slice (documented in planning/atomic-lib-runtime.md)

• mutate uses save_locally, not the Hub ingest path.
• Replica and LocalCache remain separate code paths until refactor: consolidate commit ingest, subscribe auth, and bind-twin fixtures #1274 unifies ingest options.
• sync_with_peer requires the global Iroh endpoint (peer::start), same as today.
• No NodeEvent, blob, outbox or transport abstraction; nothing stubbed.

Consumers

• refactor: consolidate commit ingest, subscribe auth, and bind-twin fixtures #1274 (CommitIngestOpts) — IngestPolicy maps onto it
• Add local-first Python and Kotlin SDKs wrapping atomic_lib #1277, Publish-ready atomic_lib Flutter SDK (precompiled binaries + CI) #1241 — server / adapter work that should bind AtomicNode instead of Db

https://claude.ai/code/session_019asLKBrBWY5ovyeCgtmdSd