Extract stats derivation model (codex)

[numnx]

🧩 Task: Extract stats derivation model

Automated task PR opened by Code UX for T06.

📋 Summary

Sprint: Sprint 142 · QS: Code Quality & Performance Audit · Task: T06

Audit and improve Code UX quality and performance by tightening existing hot-path contracts across live snapshots, execution projection, provider telemetry, dashboard render derivation, repository boundaries, MCP validation, guardrails, and documentation.

📝 Full Task Prompt

## Objective
Extract the pure derivation logic from `useStatsPageData` into a reusable view-model helper to improve testability and avoid hidden recomputation concerns in the hook.

## Scope
- Create `dashboard/src/v2/pages/stats/stats-page-view-model.ts`
- Edit `dashboard/src/v2/pages/stats/use-stats-page-data.ts`
- Edit `tests/dashboard/v2/use-stats-page-data.test.tsx`
- Create or edit `tests/dashboard/v2/stats-page-view-model.test.ts` if the suite pattern supports a focused helper test
- Update `docs/dashboard/design-system-stats.md` if the stats derivation ownership is documented there

## Implementation Requirements
1. Move token series, active-time series, wall-time series, planning usage lookup, provider/source/token segment creation, and completion confidence derivation into an exported pure helper.
2. Keep `useStatsPageData` responsible for state, fetching, and chart state only; call the pure helper inside `useMemo` with stable inputs.
3. Add focused tests for null stats, reported-only usage, mixed reported plus estimated usage, estimated-only usage, empty buckets, and custom range behavior remaining in the hook.
4. Preserve the `StatsPageData` return shape and all display strings currently consumed by Stats page components.

## Constraints
- Do not change `useProjectStats`, `useUsageChartState`, or backend stats APIs.
- Do not add external chart or state-management libraries.
- Keep date state initialization behavior unchanged.
- Do not modify unrelated Stats page layout or styling.

## Verification
- Run `pnpm exec vitest run tests/dashboard/v2/use-stats-page-data.test.tsx tests/dashboard/v2/stats-page-view-model.test.ts` if the new helper test file is created; otherwise run the existing hook test file.
- Run `pnpm run typecheck:dashboard`.
- Success means stats derivation has pure helper coverage and the hook’s public result remains backward compatible.

✅ QA Review

⏳ QA review runs after this PR is opened. Check the task card in the dashboard for the outcome once it lands.

🤖 Model & Provider

Provider	Codex CLI
Model	gpt-5.5

⏱️ Timing

Started	2026-07-06 18:05:06 UTC
Finished	2026-07-06 18:09:33 UTC
Duration	4m 27s

📊 Token Usage

Metric	Value
Input tokens	85,817
Cached input tokens	478,976
Output tokens	6,920
Total tokens	571,713
Invocations	1
Tool calls	25
Active time	3m 13s

Included usage estimate (subscription/local login): $0.88

🌿 Branch Info

Base: feature/CODUX-142-qs-code-quality-performance-audit-6
Head: task/feature-codux-142-qs-code-quality-performance-au-t06-codex-mr9j5pi5

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🤖 Generated by Code UX