feat(appkit): vendor TaskFlow engine and add TaskflowService skeleton

[ditadi]

🥞 Stacked PR

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• stack/taskflow/service-registry [Files changed]
  • stack/taskflow/sql-warehouse-split [Files changed]
    • stack/taskflow/taskflow-service [Files changed]

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Bootstraps a third core service (this.taskflow, peer of this.cache and this.telemetry) backed by the vendored Rust+napi engine at packages/appkit/vendor/taskflow/. The service is wired into the CoreServiceRegistry introduced in #374 so existing createApp(...) callers transparently gain durable execution plumbing, with an opt-out via createApp({ taskflow: false }).

Scope of this PR (engine plumbing only)

• Vendored binaries (taskflow-darwin-arm64.node, taskflow-linux-x64.node), the JS loader, type declarations, and VENDOR.json with SHA-256 digests for the optional integrity check (APPKIT_VERIFY_TASKFLOW_VENDOR=1).
• TaskflowService wraps the vendor Engine with lifecycle hooks (initialize, factory, shutdown) and pass-through methods (start, subscribe, reconnect, resume, stop, simulateCrash). Default storage: SQLite at .appkit/taskflow/. Emits a warn on Databricks Apps + SQLite (multi-pod / ephemeral filesystem; tasks would not survive rolling restarts).
• Plugin base class exposes this.taskflow (TaskflowService | null) and requireTaskflow() for the strict-mode caller. The deprecated taskflow legacy field on BasePlugin.attachContext lands here, paired with the same deprecation flags for cache? / telemetry? that the registry restructure (refactor(appkit): introduce CoreServiceRegistry and ServiceLocator #374) already implied.
• ServerPlugin._gracefulShutdown now consumes the shutdownCoreServices callback that _createApp already passes, draining cache/telemetry/taskflow through the registry. The SIGTERM/SIGINT re-entrancy guard is intentionally deferred to the hardening pass.
• tsdown.config.ts copies the vendor files into dist/appkit/vendor/ so the published package's emitted loader resolves the .node binary at the expected relative path.
• .gitignore (root + dev-playground + template) excludes the per-machine SQLite/WAL files under .appkit/.
• knip.json skips packages/appkit/vendor/** (vendored module exports are not consumed by AppKit's own code).

Deliberately NOT in this PR (follow-up)

• task() registration, TaskDefinition, TypedTaskContext, TaskRef, the step() helper.
• Plugin.executeTask SSE bridge (the bridge file, ActiveBridge, _registerBridge, the shutdown drain loop).
• Re-exports for SseEvent, TASKFLOW_IK_HEADER, setupSseHeaders, writeSseFrame, ExecuteTaskSettings.
• The tools/test-helpers.ts stub service + SSE parser upgrade.
• Analytics-plugin migration; durable-task example app.

Plugins can call this.taskflow.start(name, input) and subscribe via this.taskflow.subscribe(idempotencyKey), but no task can be defined yet — task() registration arrives with the typed surface in the next PR.

Verified: pnpm -r typecheck, pnpm build, pnpm exec biome check, pnpm exec knip, full pnpm test (122 files, 2279 tests; +3 from #375) all green.

Signed-off-by: ditadi victordperd@gmail.com