decompose architecture into 38 atomic tasks across 12 parallel generations

Decompose the reviewed architecture specs into taskgraph-managed tasks:
- 2 setup tasks (project init, test infrastructure)
- 4 schema tasks (enums, node attrs, edge attrs, graph schemas)
- 1 error hierarchy task
- 6 graph tasks (FlowGraph class, 3 construction paths, queries, validation)
- 5 analysis tasks (type-compat, build-type-edges, ordering, template-validation, defaults)
- 5 component tasks (Operation, Sequential, Parallel, Conditional, Map)
- 2 host config tasks (GraphologyHostConfig, ReactiveHostConfig)
- 4 reactive tasks (WorkflowRoot, node-status, max-concurrency, retry-semantics)
- 3 review tasks (foundation, reactive-and-hosts, complete-library)
- 5 meta cluster tasks (schema, graph, component, reactive, analysis layers)
- 1 API exports task

Validated with taskgraph: zero cycles, 38 tasks, 12 parallel generations.
Critical path: 12 tasks through reactive execution layer.

tasks/reactive-max-concurrency.md

@@ -0,0 +1,38 @@
+---
+id: reactive/max-concurrency
+name: Implement maxConcurrency semaphore for Parallel groups
+status: pending
+depends_on:
+  - reactive/node-status
+  - component/parallel
+scope: narrow
+risk: medium
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement the reactive counting semaphore that enforces `maxConcurrency` on Parallel groups. When the root initializes signals for nodes in a Parallel group with `maxConcurrency: N`, a node's effective `ready` transition requires both `preconditions.value === true` AND `runningCount < maxConcurrency`.
+
+## Acceptance Criteria
+
+- [ ] For each node in a Parallel group with `maxConcurrency`, the `canStart` computed wraps preconditions: `preconditions.value && runningSiblingCount < maxConcurrency`
+- [ ] `runningSiblingCount` is a reactive computed derived from counting sibling nodes currently in `"running"` state
+- [ ] When a sibling completes and a slot opens, the next ready node transitions to ready/startable
+- [ ] Parallel groups without `maxConcurrency` (default): all siblings start immediately when preconditions met — no semaphore
+- [ ] `maxConcurrency` is a runtime scheduling hint — the DAG doesn't encode it
+- [ ] Unit tests: parallel group with maxConcurrency: 2 limits to 2 running at a time, slot opens → next node starts, no maxConcurrency → all start immediately
+
+## References
+
+- docs/architecture/reactive-execution.md — maxConcurrency, counting semaphore
+- docs/architecture/workflow-templates.md — Parallel.maxConcurrency prop
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion