[quickbeard]

Summary

Follow-up to #380. That PR fixed the host-arch helper build used by bun dev:desktop. Release packaging (prebuild.ts → build-vzhelper.ts --universal) still failed on Command Line Tools 27 / Swift 6.4, so macOS releases had to be built by hand.

Why the Intel slice failed. The helper's .macOS(.v12) deployment target links Swift's back-compatibility libraries:

• libswiftCompatibility56 supplies runtime fixes for macOS 12.0–12.2.
• libswiftCompatibilityPacks also gets linked.

This toolchain ships every compatibility library as arm64 arm64e only (checked with lipo -archs on all six), so the x86_64 link failed:

ld: warning: ignoring file …/libswiftCompatibility56.a: fat file missing arch 'x86_64', file has 'arm64,arm64e'
ld: warning: ignoring file …/libswiftCompatibilityPacks.a: fat file missing arch 'x86_64', file has 'arm64,arm64e'
Undefined symbols for architecture x86_64

Fix. When the toolchain's libswiftCompatibility56.a has no x86_64 slice, the Intel slice is built with -Xswiftc -runtime-compatibility-version -Xswiftc none, which leaves the compatibility libraries out.

• The arm64 slice is unchanged and keeps them.
• A toolchain that still ships Intel copies (e.g. Xcode 26 on CI) keeps them for Intel too.
• The only cost is on Intel Macs running macOS 12.0–12.2 (sandboxes require macOS 12+), where the helper runs without those runtime fixes. Every other Mac is unaffected.

It also quiets the first swift build attempt before the --build-system native retry from #380. A successful build used to log the default backend's full error; now it logs one line, and a failing retry still prints its own error.

packages/codev-sandbox is fork-only, so no DIVERGENCES.md row.

Test plan

• Clean --universal build on CLT 27 / Swift 6.4 (.build and dist removed first): succeeds in ~70 s. lipo -archs → x86_64 arm64, signed with com.apple.security.virtualization.
• nm per slice: arm64 still carries the swiftCompatibility symbols (2), x86_64 has none.
• Host-arch build (the predev path) unchanged: arm64 only.

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