ci: Swift 6.2/6.3 matrix, GitHub Actions node24, Xcode 26.4

[Sweefties]

Description

Modernize CI to support Swift 6.2 and 6.3, migrate all GitHub Actions to Node 24, and bump Xcode 26.4 on platform-builds, release, and DocC jobs.

Changes

• ci.yml: build-and-test matrix with Swift 6.2 (Xcode 26.2) + Swift 6.3 (Xcode 26.4), coverage/Codecov on 6.2 only
• ci.yml: platform-builds bump Xcode 26.2 → 26.4
• release.yml: bump Xcode 26.2 → 26.4 for macOS build
• All actions migrated to Node 24: checkout v4→v5, upload-artifact v4→v6, download-artifact v4→v7, codecov-action v5→v6, action-gh-release v2→v3, upload-pages-artifact v3→v4, deploy-pages v4→v5
• Scripts/mock-hls-server.py: include locally modified script

Checklist

• Tests added/updated
• swift build passes
• swift test passes (0 failures)
• swiftlint lint --strict (0 violations)
• Documentation updated if needed
• No breaking changes (or documented in description)

[codecov]

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