Fix fresh-checkout builds

[onionviolet]

What this fixes

Fresh checkouts could not reliably build the native library or config app without
manually preparing dependencies and package paths.

This change:

• centralizes Dobby setup in the Makefile and pins its revision;
• rebuilds and relinks Dobby when the recorded revision is stale;
• removes duplicate dependency setup from scripts/build.sh;
• declares the Swift test target only when its local test sources exist; and
• drops an SDK-only corner treatment so the config app builds with the current
  toolchain.

It does not change injection, hook, or feature behavior.

Testing

Run from a clean detached worktree based on current upstream main:

• swift package resolve --package-path macsteam-app passed and resolved Sparkle 2.9.6.
• make test passed by taking the repository's existing clean-checkout no-local-tests path.
• make -j2 passed and produced an ad-hoc-signed universal out/macsteam.dylib.
• bash macsteam-app/make_app.sh passed and produced an ad-hoc-signed arm64 app bundle.
• A deliberately stale build/.dobby-revision caused Dobby to rebuild and the arm64
  library to relink.
• git diff --check upstream/main...HEAD passed.

Known upstream warnings remain: Dobby's CMake deprecation/assert warnings and the
existing dynamic Selector warning in AppDelegate.swift.

AI assistance

I am the sole human contributor to this pull request. AI assisted substantially through
OpenAI Codex, including GPT-5.6 Sol for an independent final-diff and test review. The
review found two build-cache and empty-test-target problems, which were fixed before this
PR was opened. The clean-build commands and results above were then rerun. I have not
added any AI identity as a commit author or co-author.

This PR addresses fresh checkouts failing before the project and config app can build.
I'm happy to adapt the implementation or have portions rewritten to better fit the
project.