Bump toolchains to Swift 6.3 and Xcode 26.6

[Kyle-Ye]

Require Swift 6.3 and use Xcode 26.6 / Swift 6.3.3 in CI. Keep Swift 5 language mode and the existing public API and deployment targets.

Remove obsolete compiler branches, the manual Linux test discovery script, and old runner workarounds. Use current macOS, Ubuntu, and Windows runners, and run WebAssembly tests with the official Swift SDK instead of testing the host toolchain.

Fix Foundation callback diagnostics, inherited Sendable conformances, optimized test subscription lifetimes, and assertions about Combine's private Future layout. Keep notification mock tests on Objective-C platforms and test real notification delivery, demand, filtering, cancellation, and lifetime on all supported Foundation platforms. Compare SwiftLint violations with the PR base so existing style debt does not block unrelated changes.

Validation:

• macOS: debug coverage, release, TSan, Combine compatibility tests, library evolution, Swift module interfaces, and public OSLock build.
• Ubuntu 24.04 with Swift 6.3.3: debug, release, and TSan tests.
• WebAssembly: 1,419 tests passed on wasm32 with the Swift 6.3.3 SDK.
• SwiftLint baseline check, actionlint, generated source check, and git diff check.

All seven GitHub Actions checks pass on ee8fb56. Codecov still rejects protected-branch uploads without authentication. Tests and coverage generation pass, but upload authentication remains to be configured; the current uploader treats that rejection as a non-fatal error.