Implement portfolio orchestration API for app integrators

[andywkff]

Summary

Implement the first-class portfolio orchestration workflow for app integrators.

This work should:

• keep current single-instrument execution intact
• aggregate existing sleeve-level runs instead of introducing a shared-cash simulator
• expose both a canonical BKEngine portfolio path and a simpler BKAppFacade basket-review path
• return aggregate portfolio results plus per-sleeve outcomes/failures

Scope

• Add BKEngine.PortfolioRequest and BKEngine.runPortfolio(...)
• Add app-facing basket import/review helpers
• Add portfolio models for sleeves, allocations, rebalance policies, aggregate summaries, and structured failures
• Extend export/report helpers for stable portfolio JSON/CSV/Markdown outputs
• Add tests for orchestration semantics, façade workflows, and export adapters

Notes

• Primary audience: app integrators
• v1 should support per-sleeve config, explicit weights plus helper-derived weights, and simple periodic/manual rebalance rules
• Out of scope: shared-cash simulation, event-driven rebalance logic, persistence layers

[andywkff]

Implemented the portfolio orchestration PRD end to end.

What landed:

• Added a canonical BKEngine.PortfolioRequest surface and BKEngine.runPortfolio(...) additive orchestration path.
• Added portfolio domain models for sleeve requests, allocation modes, rebalance policies/events, per-sleeve reports, and aggregate portfolio reports.
• Added app-facing basket helpers: BKAppFacade.buildPortfolioCSVImportScreenState(...) and BKAppFacade.runConfirmedPortfolioCSVImport(...).
• Added portfolio export/report adapters for JSON, CSV, and Markdown outputs.
• Added regression coverage for aggregation, helper-driven weight resolution, rebalance semantics, partial failure/fail-fast behavior, app basket workflows, and export payloads.

Verification:

• swift build
• swift test --filter 'BacktestingKitPortfolioTests|BacktestingKitAppFacadeTests'
• swift test (122 tests, 0 failures)

Roadmap and local task artifacts were updated to reflect completion.