feat(orch8): embeddable crate — durable execution as a Rust library

[ovasylenko]

Summary

The primary strategic bet from docs/FEATURE_OPPORTUNITIES.md (Bet A, 34/40): cargo add orch8 — durable execution as a library, the way SQLite is a database.

let engine = orch8::Engine::builder()
    .storage(orch8::Storage::sqlite("app.db"))   // or sqlite_in_memory() / postgres(url)
    .handler("charge_card", |ctx: orch8::StepContext| async move {
        Ok(serde_json::json!({ "charged": true }))
    })
    .build().await?;
engine.start();                                   // tick loop on the HOST's tokio runtime

• Facade crate (publish = false for now): curated re-exports + Engine/EngineBuilder/Storage/CreateInstanceOptions/Error — internals stay free to churn behind it. Zero visibility changes were needed in internal crates — the engine was already embeddable.
• build() mirrors server startup (schema/migrations + stale-instance recovery); shutdown() cancels and drains; tick_once() supports manual-tick embedding for test harnesses.
• upsert_sequence is create-or-get per (tenant, namespace, name, version) — versions stay immutable so running instances stay pinned.
• Three examples, all manually verified: quickstart (sqlite + custom handler to completion), kill_resistant (run twice — the surviving instance resumes after process death), embedded_axum (HTTP service sharing the engine).

Tests

6 passing in orch8/tests + doc-test: background-loop completion, manual tick, signal-gated wait + wake, bounded graceful shutdown, CRUD/idempotency semantics. fmt/clippy/doc/check gates clean workspace-wide.

Caveats

shutdown() is terminal (documented); postgres wired but not CI-exercised (no PG harness in repo); list_instances fixed newest-first/100 (minimal pre-1.0 surface).

🤖 Generated with Claude Code