feat: honour --targets app on macOS for app-only builds

[kud]

What

On macOS, pake <url> --targets app always produced a .dmg instead of an app-only build. This makes --targets app actually build only the .app bundle, skipping the DMG packaging step.

Why

merge.ts already treats app as a valid macOS bundle target (validMacTargets = ['app', 'dmg']), but MacBuilder's constructor set buildFormat to dmg for any value that wasn't an arch — and then overwrote options.targets with it before mergeConfig could apply the app target. So the app branch in merge.ts was unreachable.

Building the DMG also mounts a volume window and adds noticeable time, which is unnecessary when you only want the .app.

How

Treat an explicit --targets app the same way --install and iterativeBuild already request the app format in the constructor.

Tests

Adds tests/unit/mac-builder-targets.test.ts: --targets app yields an app-only filename; dmg/default/arch values still resolve to DMG builds. Full suite: 174 passing.

Docs

Documents app/dmg as macOS output formats in docs/cli-usage.md and docs/cli-usage_CN.md.