Keep builds and uploads working across compiler and metadata updates

[Str-Gen]

Summary

Keep package builds and uploads moving when recipes use different Mojo compiler versions or another runner has already committed the same compatibility metadata.

 build each recipe:
-    share output/ as the highest-priority dependency channel
+    use output/<recipe>/ so other recipes cannot shadow published dependencies

 git pull origin main
+if nothing remains staged:
+    return success
 git commit
 git push

The Linux failure in run 39267609540 occurs when Pyroquet resolves its dependencies: the locally rebuilt mojo-snappy 0.2.0 requires Mojo 1.1.0, while Pyroquet requires Mojo 1.0.0. Strict channel priority hides the compatible published mojo-snappy build. Isolate each recipe's output channel; the upload helper already discovers packages recursively. Keep Pyroquet's existing pins while its dependency stack awaits published Mojo 1.1-compatible packages.

The earlier macOS failure in run 39098506151 was an empty metadata commit after pulling an identical upstream update. Recheck the staged diff after pulling, preserving errors from the diff command.

Evidence

• Before: A local-channel regression runs the real rattler-build solver through scripts.build-all and fails with the same strict-channel-priority / Mojo 1.1 dependency error.
  After: The same regression passes with per-recipe output directories. A second test verifies recursive upload discovery.
• The three Git integration tests cover identical concurrent updates, new updates committed and pushed, and unchanged data.
• All five tests pass with pixi run python -m unittest discover -s tests -v; repository pre-commit checks pass using Pixi 0.81.0 and the locked environment.
• Full package compilation remains subject to the updated PR's Linux/macOS/ARM CI checks.

Merge Danger

Door: Two-way; both changes can be reverted.

Blast Radius: CI. Build artifacts now live under output/<recipe>/<platform>/. Recipes resolve dependencies from published or explicitly supplied channels instead of implicitly consuming another recipe's output from the same invocation. This prevents build order from selecting incompatible compiler builds, but dependencies that are not published yet still require an explicit channel. The existing recursive upload helper supports the new layout. No recipe or dependency version changes. The metadata fix handles empty commits, not every concurrent push race.

After merge, verify that the build/upload workflow publishes the mojo-snappy 0.2.0 build 1 rebuild from #408, or trigger it on main if needed.

[Str-Gen]

Mojo-snappy was built against Mojo 1.1.0 with changes to conform to the changelog. However, it is used in pyroquet because parquet uses snappy compression (among others). However, my pyroquet package also imports NuMojo and they don't have a release against Mojo 1.1.0 yet so for now this is blocked but PR 412 should solve this and then both mojo-snappy and pyroquet can be built and distributed through prefix.dev.
I'll go through a similar process then for Balsa and Pyre (Treelite v4 reader / writer) & accelerated forest inference library
No action is needed for now. I'm still figuring out the workflow.