fix: serialize visible columns deterministically

[MattKotsenas]

The order of visible_columns in pstoprc changes nondeterministically between saves, producing no-op diffs that make the file noisy to track in version control.

Summary

• sort visible column indices before writing pstoprc
• extract PstopConfig::serialize() so config output is testable without touching %APPDATA%
• add a regression test that pins visible-column membership and canonical order

Runtime behavior is unchanged: column visibility remains set-based, and display order comes from the static column definitions. Existing config files are rewritten in canonical order on the next save.

Testing

• before sorting, the targeted test failed with visible_columns=16,0,9
• after sorting, cargo test visible_columns_are_serialized_in_canonical_order passes