feat: add performance budget review for Aegis Dashboard (#44)

[anthonia01-dev]

Description

Implements the performance budget review feature for the Aegis Dashboard (Issue #44).

Changes

Core Logic

• src/lib/performanceBudget.ts — Typed data model and pure functions for performance budget review:
  • BudgetCheck, BudgetReviewResult, BudgetReviewState types
  • deriveBudgetStatus — maps check results to compliant | warning | breached | unknown
  • filterBudgetResults, tallyBudgetResults, validateBudgetChecks
  • Bulk actions: approve | reject | flag-for-review | clear
  • budgetResultLabel with compliance-safe disclaimer integration

Tests

• src/lib/performanceBudget.test.ts — 30 unit tests covering all status derivations, custom rules, filtering, tally, selection helpers, bulk actions, value formatting, label generation, and check validation

Fixtures

• src/lib/fixtures/performanceBudget.ts — 4-row sample queue exercising all statuses

UI Component

• src/features/diagnostics/components/PerformanceBudgetPanel.tsx — Reusable table with filtering, sorting, saved views, bulk action bar with confirmation flow, and COMPLIANCE_DISCLAIMER on header and action confirmations

SDK Integration

• Extended IAegisProvider with getPerformanceBudget method
• Implemented in MockAegisProvider and LiveAegisProvider
• Mock client in src/lib/aegis/client.ts

Feature Flag

• Added performanceBudgetReview flag in useFeatureFlags

Documentation

• docs/performance-budget-review.md — Full contributor guidance with data model, behaviour, edge cases, security assumptions, tests/fixtures checklist, and compliance-safe wording guidance
• Updated README.md and docs/README.md with links

Acceptance Criteria

• ✅ Performance budget review is implemented with typed behaviour
• ✅ Relevant edge cases and failure states are handled (empty checks, unknown results, no selection, duplicate keys, missing fields)
• ✅ Security and compliance-sensitive assumptions are documented
• ✅ Tests (30) and fixtures added
• ✅ README/docs link to the new guidance
• ✅ Compatible with the rest of the Aegis ecosystem (SDK abstraction, feature flags, compliance disclaimer)

Verification

• All 455 tests pass
• Build succeeds with no errors
• No lint errors

Closes #44