`unable to find sdk '<platform>'` when building Swift package against a Swift SDK with target-platform specialization

[MaxDesiatov]

When SwiftPM (swiftbuild build system) builds a multi-target Swift package against a Wasm Swift SDK, the build fails during dependency resolution / settings construction with:

error: unable to find sdk 'webassembly'
warning: ONLY_ACTIVE_ARCH=YES requested with multiple ARCHS and no active architecture could be computed; building for all applicable architectures
error: Build failed

The same root cause should affect any platform that has only a Swift SDK installed (no built-in or platform-extension-registered SDK with the platform's canonical name) — Wasm is the most exposed today because SWBWebAssemblyPlatform does not register a fallback SDK the way SWBAndroidPlatform's AndroidSDKRegistryExtension does.

This is the follow-up to #1267 / #1266: addRunDestinationSettingsPlatformSDK is one of two sites that push the platform-name canonical-SDK string for Swift-SDK-backed builds.

Reproduction

https://github.com/swiftwasm/JavaScriptKit, commit ff90c3e:

TOOLCHAINS=<recent-main-snapshot> \
  swift package --swift-sdk swift-DEVELOPMENT-SNAPSHOT_wasm \
  --traits Tracing --disable-sandbox \
  js test --build-only --prelude ./Tests/prelude.mjs

Building for debugging...
Computing dependencies
Provisioning 19 / 60
error: unable to find sdk 'webassembly'
warning: ONLY_ACTIVE_ARCH=YES requested with multiple ARCHS and no active architecture could be computed; building for all applicable architectures
error: Build failed

(Workaround: --build-system native succeeds — the legacy build system is unaffected.)