Downgrade swift-tools-version to 6.0 for broader compatibility (#3)

Summary:
This PR makes QuickLayout accessible to developers using stable Xcode versions by downgrading the Swift tools version requirement.

## Changes

- Downgrade `swift-tools-version` from 6.2 to 6.0
- Update `swift-syntax` dependency from 602.0.0 to 600.0.0
- Fix Demo project iOS deployment target from 26.0 to 15.0

## Rationale

- **Swift 6.2 requires Xcode 16.5 Beta**, which most developers don't have access to. Swift 6.0 (Xcode 16.0+) is widely available.
- **swift-syntax version must match the Swift tools version** (600.x for Swift 6.0).
- **iOS 26.0 does not exist** - this was a configuration error. Changed to 15.0 to match the `platforms` declaration in Package.swift.

## Verification

- [x] `swift package resolve` succeeds
- [x] `xcodebuild -scheme QuickLayout -destination 'platform=iOS Simulator,name=iPhone 16' build` succeeds
- [x] Demo project builds successfully

Pull Request resolved: #3

Reviewed By: tumtumtum

Differential Revision: D88010997

Pulled By: constantine-fry

fbshipit-source-id: 751cfb12ccdae289dae222a383aeff3962e14bb2

Author: JonyFang

Demo/Demo.xcodeproj/project.pbxproj

@@ -199,7 +199,7 @@
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				IPHONEOS_DEPLOYMENT_TARGET = 26.0;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.0;
 				LOCALIZATION_PREFERS_STRING_CATALOGS = YES;
 				MTL_ENABLE_DEBUG_INFO = INCLUDE_SOURCE;
 				MTL_FAST_MATH = YES;
@@ -256,7 +256,7 @@
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				IPHONEOS_DEPLOYMENT_TARGET = 26.0;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.0;
 				LOCALIZATION_PREFERS_STRING_CATALOGS = YES;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				MTL_FAST_MATH = YES;

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 6.2
+// swift-tools-version: 6.0
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 /*
  * Copyright (c) Meta Platforms, Inc. and affiliates.
@@ -35,7 +35,7 @@ let package = Package(
     ),
   ],
   dependencies: [
-    .package(url: "https://github.com/apple/swift-syntax.git", from: "602.0.0")
+    .package(url: "https://github.com/apple/swift-syntax.git", from: "600.0.0")
   ],
   targets: [
     .target(