Fix watchOS build failure on Xcode 27

[maximbilan]

Problem

The package fails to compile for watchOS with Xcode 27 beta:

Sources/EventSource/EventParser.swift:91:25: error: 'split(by:)' is unavailable in watchOS:
This method is not recommended on watchOS 9.0+
note: 'split(by:)' was obsoleted in watchOS 9.0

Cause

Xcode 27 raises the minimum supported deployment targets, so the floors declared in Package.swift get clamped upward:

Platform	Xcode 26 min	Xcode 27 min	obsoleted: at	Result
watchOS	4.0	9.0	9.0	error
iOS	12.0	15.0	16.0	ok
tvOS	12.0	15.0	16.0	ok
macOS	10.13	12.0	13.0	ok

Package.swift declares .watchOS(.v6), so Xcode 26 compiled with -target arm64-apple-watchos6.0 and the fallback was legal. Xcode 27 clamps to watchos9.0, which meets obsoleted: 9.0 exactly and makes the symbol a hard error.

Being inside the else of if #available(..., watchOS 9.0, *) does not rescue it — an #available else branch does not lower the availability context, so the call is still checked against the deployment target.

Fix

Remove the @available attributes from the fileprivate fallback and rename it legacySplit(by:). They were decorative on a method nothing outside the file could call, but they broke the build. This also removes the need for the #if os(visionOS) special case added in 1c45c73, which worked around the same root cause on a different platform.

iOS and macOS are only one OS release away from the same collision (clamped to 15.0/12.0 against obsoleted: 16.0/13.0), so removing the annotations rather than bumping them should prevent a repeat on the next toolchain.

Deployment targets in Package.swift are unchanged — not a breaking change for consumers.

Verification

• Clean builds on watchOS, iOS, tvOS, visionOS, macOS under Xcode 27.0 beta (27A5252f)
• Still builds under Xcode 26.6
• All 7 tests pass

One note: the fallback path isn't exercised by the test suite — #available takes the modern branch on the test host — but the body is byte-identical to before, only renamed.

🤖 Generated with Claude Code

[Recouse]

Thank you!