[Context]: add DAG orchestration ContextGraphBuilder

[rian-be]

Summary

Add a DAG orchestration extension: ContextGraphBuilder + ExecuteGraphAsync to declare task dependencies and let the library derive which tasks can run in parallel and which must wait for predecessors.

Goal

Orchestrate many context-scoped tasks with declared dependencies, computing parallelism automatically — as an Extension, not part of Core.

Problem

For fan-out/fan-in flows (fetch → validate → save), manual Task.WhenAll ordering is brittle. There is no declarative dependency model.

Scope

var graph = new ContextGraphBuilder<ShopContext>()
 .AddNode("fetch_order", () => FetchOrder(id))
 .AddNode("fetch_customer", () => FetchCustomer(id))
 .AddNode("validate", ValidateOrder, dependsOn: new[] { "fetch_order", "fetch_customer" })
 .Build();

await manager.ExecuteGraphAsync(ctx, graph);

• Node definition with dependsOn edges.
• Cycle detection (fail on cycles).
• Topological scheduling within a single context scope; independent nodes run concurrently.
• Per-node context isolation + lifecycle events.
• Cancellation + progress support.
• Unit tests for parallelism and ordering.

Design Expectations

• Kept minimal; lives at the Extension level (per Core-minimal-primitive rule).
• No core runtime changes; orchestration runs topologically over base primitives.

Acceptance Criteria

• Independent nodes run in parallel; dependent nodes wait.
• Cycles are rejected.
• Runs within one logical context with correct isolation.

Non-Goals

• No workflow-engine features (state, persistence, retry / Polly).
• No multi-process/queue orchestration (adapter-level).