Fix expression caching within polling attempts

[GtechGovind]

Fixes #360.

I reproduced a polling failure where the matcher result and diagnostic observed different values from the same expression. The expression could be evaluated more than once during a single polling attempt.

I added a fresh expression cache for each polling attempt across synchronous, async, and require expectations. Matchers reuse a value within an attempt, while subsequent attempts still observe changes. The async-to-synchronous conversion already materializes its expression once, so I left that path unchanged.

I added regressions for changing expressions, async matchers that evaluate twice, returned require values, and continuous always/never polling, and documented the caching boundary.

Validation:

• Focused polling matrix: 89 tests passed.
• New async/require regressions: 3 passed; continuous-polling regression: 1 passed.
• Full SwiftPM: 383 Swift and 94 Objective-C tests passed, plus Swift Testing support checks with their expected known issues.
• macOS Xcode tests: 488 passed, 4 expected failures.
• iOS simulator Xcode tests: 488 passed, 4 expected failures, including the continuous-polling regression.
• git diff --check passed.

Installed Xcode required command-only deployment-target overrides (macOS 12 and iOS 15); I did not change the project's supported versions. Other platforms and the repository's CI toolchain matrix remain for CI.