Decompose architecture into 28 atomic implementation tasks

Break the @alkdev/taskgraph architecture specs into dependency-ordered
implementation tasks across 8 component directories: setup, schema,
error, graph, analysis, cost-benefit, frontmatter, api, and review.
Each task has clear acceptance criteria referencing specific architecture
docs. Three review tasks serve as quality gates at critical junction
points (schemas-and-errors, graph-complete, complete-library). The
dependency graph is validated acyclic with 9 topological levels enabling
significant parallelism across independent work streams.

tasks/implementation/setup/test-infrastructure.md

@@ -0,0 +1,42 @@
+---
+id: setup/test-infrastructure
+name: Configure test runner and shared test fixtures
+status: pending
+depends_on:
+  - setup/project-init
+scope: narrow
+risk: low
+impact: project
+level: implementation
+---
+
+## Description
+
+Set up the test infrastructure: configure Vitest (or chosen runner), create shared test fixtures and helpers for graph construction that all downstream test files will use. This avoids every test file building graphs from scratch.
+
+## Acceptance Criteria
+
+- [ ] Test runner configured in `package.json` scripts (`"test"`, `"test:watch"`, `"test:coverage"`)
+- [ ] Vitest config (or equivalent) exists with ESM support and TypeScript path resolution
+- [ ] Shared test fixture file created (e.g., `test/fixtures/graphs.ts`) with:
+  - A simple linear chain graph (3-4 tasks, A→B→C→D)
+  - A diamond dependency graph (A→B, A→C, B→D, C→D)
+  - A graph with mixed categorical fields (some assessed, some null)
+  - A graph with cycles for testing cycle detection
+  - A larger graph (20+ nodes) for performance/bottleneck testing
+- [ ] Helper function to create a `TaskGraph` from `TaskInput[]` for one-liner test setup
+- [ ] Test runner executes successfully against placeholder test files
+- [ ] CI-compatible output format (no watch mode in default script)
+
+## References
+
+- docs/architecture/build-distribution.md — test directory structure
+- docs/architecture/graph-model.md — graph construction examples for fixtures
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion