Split native host structure and document Swift boundary

[yvette-carlisle]

Summary

• split frozen capture models, capture overlay controller, and capture host view out of NativeHostApp.swift while preserving behavior
• keep NativeHostApp.swift focused on app/session orchestration and host/core bridge routing
• document the remaining Swift responsibility map and the next migration/optimization order

Verification

• cargo make fmt-swift-check
• cargo make check-swift
• cargo make test-host-reset
• cargo make check-vstyle-swift
• git diff --check
• scripts/build_and_run.sh --verify

Semantic drift: manual pass against docs/spec/platform-host-boundary.md, current Swift host files, and Rust FFI/core anchors.