CI: bump device-platform simulator OS back to 26.5 once macos-26 runner image refreshes

[leogdion]

Context

PR #156 (research/swift-manifest-codegen) needed to temporarily revert the
device-platform simulator pin in .github/workflows/SyntaxKit.yml from
osVersion: "26.5" back to "26.4" for iOS / watchOS / tvOS / visionOS.

Surfaced by failing run 25759444849:
the macos-26 runner image only ships Xcode 26.4.1 and OS 26.4 simulators
right now, so download-platform: true couldn't fetch OS 26.5 runtimes for
the iOS / visionOS / watchOS device names in the matrix. tvOS happened to
work for OS 26.4 because Apple TV is in the runner image's default
simulator set; the others aren't.

Acceptance

Once GitHub refreshes macos-26 to include Xcode 26.5 (track upstream via
actions/runner-images):

1. Flip osVersion: "26.4" → "26.5" at the four lines in
   .github/workflows/SyntaxKit.yml:
   • iOS matrix entry
   • watchOS matrix entry
   • tvOS matrix entry
   • visionOS matrix entry
2. Confirm the full PR matrix passes.
3. Close this issue.

The Xcode pins (Xcode_26.5.app) on those rows are unchanged from the
revert — brightdigit/swift-build@v1 already falls back to whatever Xcode
the runner ships. Only the osVersion was reverted.

Related

• The Windows 6h-timeout jobs in the same failing run are a separate
  upstream issue (brightdigit/swift-build@v1 / runner-image churn around
  windows-2025 → windows-2025-vs2026). Not blocked on this issue.