Restore dual architecture support for 3.9

[kieran-osgood-shopify]

What changes are you making?

• Restores @shopify/checkout-sheet-kit to the 3.9.0 line while preserving the v3 public async API.
• Keeps the JavaScript API compatible with both TurboModule and legacy NativeModules resolution.
• Restores old/new architecture support on Android and iOS by gating codegen/new-arch dependencies and exporting legacy bridge Promise methods.
• Updates docs, sample usage, and tests for the 3.9 dual-architecture migration release.

Validation

• pnpm module typecheck
• pnpm module lint
• pnpm module build
• pnpm sample typecheck
• pnpm test --watchman=false modules/@shopify/checkout-sheet-kit/tests
• Android sample old arch: ./gradlew assembleDebug --no-daemon --console=plain -PreactNativeArchitectures=arm64-v8a -PnewArchEnabled=false
• Android sample new arch: ./gradlew assembleDebug --no-daemon --console=plain -PreactNativeArchitectures=arm64-v8a -PnewArchEnabled=true
• iOS sample old arch: RCT_NEW_ARCH_ENABLED=0 bundle exec pod install --no-repo-update plus generic simulator xcodebuild clean build
• iOS sample new arch: RCT_NEW_ARCH_ENABLED=1 bundle exec pod install --no-repo-update plus generic simulator xcodebuild clean build

[kieran-osgood-shopify]

• Restore dual architecture support for 3.9 #487 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[github-actions]

Coverage Report

Lines	Statements	Branches	Functions
	99.12% (227/229)	94.69% (125/132)	100% (68/68)