Audit Swift 6.3 APIs and compiler diagnostics

[andywkff]

Audit BacktestingKit with the latest stable Swift 6.3.3 toolchain, run Swift 6.4 beta as a forward-compatibility check, fix confirmed API changes plus compiler errors and warnings, and record clean build/test evidence. No assignment or project placement.

[andywkff]

Completed the Swift compatibility audit.

• Swift 6.3.3 configured and strict-mode test suites: 127/127 passed
• Swift 6.4 beta strict-mode test suite: 127/127 passed
• Xcode 27 beta Swift 6 iPhone 16 Pro Max simulator build: succeeded
• Implemented strict-concurrency Sendable fixes and synchronized the existing mutable one-liner hooks
• No renamed or deprecated API migration was required

One Xcode 27 beta tool warning remains: App Intents metadata extraction is skipped because the target does not depend on AppIntents.framework. The source has no App Intents dependency, and the disable setting did not suppress the beta warning.