Implement portfolio orchestration API for app integrators

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

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

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

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.