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/cost-benefit/ev-calculation.md

@@ -0,0 +1,48 @@
+---
+id: cost-benefit/ev-calculation
+name: Implement calculateTaskEv pure function
+status: pending
+depends_on:
+  - schema/numeric-methods-and-defaults
+  - schema/result-types
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement `calculateTaskEv(p, scopeCost, impactWeight, config?): EvResult` in `src/analysis/cost-benefit.ts`. This is the pure math function — takes numeric inputs, returns EV result. No graph dependency.
+
+Per [cost-benefit.md](../../../docs/architecture/cost-benefit.md):
+```
+EV = P_success × C_success + (1 - P_success) × C_fail
+```
+Where `C_fail = scopeCost + fallbackCost + timeLost × expectedRetries`.
+
+## Acceptance Criteria
+
+- [ ] `calculateTaskEv(p: number, scopeCost: number, impactWeight: number, config?: EvConfig): EvResult`
+- [ ] Returns `EvResult`: `{ ev, pSuccess, expectedRetries }`
+- [ ] `expectedRetries` = `(1 - p) / p` when `p > 0`, else 0 (geometric series)
+- [ ] `C_fail = scopeCost * impactWeight + fallbackCost + timeLost * expectedRetries` (impactWeight scales the cost)
+- [ ] `EV = p * scopeCost * impactWeight + (1-p) * C_fail`
+- [ ] When `config.retries` is provided and > 0, caps `expectedRetries` at `retries`
+- [ ] When `config.valueRate` is non-zero, multiplies the final EV
+- [ ] Edge cases: `p = 0` (guaranteed failure), `p = 1` (guaranteed success), default config (all zeros)
+- [ ] Unit tests: known calculations from the Python research model, boundary values, config variations
+
+## References
+
+- docs/architecture/cost-benefit.md — EV formula, EvConfig parameters, EvResult
+- docs/architecture/api-surface.md — calculateTaskEv signature
+- docs/architecture/schemas.md — EvConfig, EvResult schemas
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion