Refine Runtime Port Adapters

[Pigbibi]

Summary

This PR tightens the shared runtime contract that the broker runtimes now rely on for orchestration decoupling.

The immediate issue was that platform repos were already moving toward PortfolioPort / ExecutionPort / MarketDataPort based wiring, but the shared translation helpers still dropped some account metadata on the floor. In particular, cash_by_currency and sellable_quantities could be lost when converting between broker-native account state and PortfolioSnapshot, which makes cross-platform orchestration less predictable and forces platform code to keep broker-specific fallbacks around.

Root cause

The shared runtime contract had the right high-level models, but the snapshot/account-state conversion path was still too lossy for the newer orchestration shape. At the same time, the platform-boundary guidance did not yet document the newer adapter/composer layering that the runtimes are now using.

What changed

• added quant_platform_kit.common.port_adapters so runtimes can bind plain callables to shared ports with a thin local adapter layer
• preserved sellable_quantities when building value-target portfolio inputs from a PortfolioSnapshot
• preserved both cash_by_currency and sellable_quantities when translating between PortfolioSnapshot and account_state
• extended contract tests to lock in the metadata round-trip behavior
• updated platform_repo_boundaries.md with the local builder/composer guidance used by the platform repos

Validation

• python -m pytest tests/test_common_port_adapters.py tests/test_strategy_contracts.py