Tolerate missing juliaupself.json

[ghyatzo]

read_config_db and load_mut_config_db previously hard-bailed on any error opening juliaupself.json, including NotFound. This is inconsistent with the main config (juliaup.json), which returns JuliaupConfig::default() on NotFound.

The asymmetry meant that a missing juliaupself.json would:

• prevent Julia from launching (launcher -> load_config_db_lockfree -> bail)
• prevent juliaup from running any mutating command (load_mut_config_db -> bail)
• prevent juliaup self uninstall from running (trapped users)

This is also the single biggest risk for any future install-layout change, and is independently a fragility bug.

Fix:

• read_config_db: treat NotFound as JuliaupSelfConfig::default()
• load_mut_config_db: on NotFound, create the file with default contents and proceed normally (bootstraps the self-config)
• Add Default derive to JuliaupSelfConfig

[IanButterworth]

Codex findings:

• P1 — Create juliaupself.json atomically. src/config_file.rs:555 creates/truncates the destination before serializing. Because the launcher reads configuration without locking, it can observe an empty/
  partial file and fail. A crash or write error also leaves a corrupt file that this PR does not tolerate. Use the existing temp-file, flush/sync, atomic-rename pattern and then reopen it.

• P2 — Missing config can leave the background updater after uninstall. src/config_file.rs:546 defaults background_selfupdate_interval to None. During self uninstall, that equals the requested disabled
  state, so src/command_config_backgroundselfupdate.rs:29 skips uninstall_background_selfupdate(). If the deleted config previously enabled the cron/scheduled task, uninstall leaves it behind. Cleanup
  should be unconditional when uninstalling or when configuration state is unknown.

• P2 — The changed behavior has no regression coverage. The config tests are excluded when selfupdate is enabled at src/config_file.rs:788, while the self-update fixture explicitly creates the file at
  tests/command_selfupdate_test.rs:146. Add tests for lock-free reads with a missing self-config and atomic mutable bootstrap.

CI also currently fails rustfmt at src/config_file.rs:567; cargo fmt resolves that.

[ghyatzo]

Should have addressed P1 (copied the normal juliaup.self mechanism also for the selfconfig file), not sure what's the suggestion for P2 though. I find that wording a bit confusing.