Exclude Xbox / Game Pass games by default (#494)

[mgth]

Fixes #494.

Xbox / Game Pass titles install under …\XboxGames\…\Content\ and keep sliding across monitors even in fullscreen, because the default exclusion list only covered Epic / Steam / Riot.

Changes

• ExcludedProcessDefaults (new, Core) — single source of truth for the default exclusions (Epic, Steam, Riot, \XboxGames\), with a Version and the pre-v1 LegacyV0 set. Shared by the seed file, the migration and the UI.
• Seed — CreateExcludedFile writes from ExcludedProcessDefaults instead of a hardcoded string, so fresh installs get Xbox.
• One-time top-up migration (PersistencyExtensions.LoadOptions) — reaches existing users, not just fresh installs. Gated by a registry marker (ExcludedDefaultsVersion) so it runs once; only tops up a list that still holds all previous defaults (a customized list, or a default removed later, is left alone); rewrites Excluded.txt since the daemon reads the file, not the in-memory list.
• "Add defaults" button (options) — adds any missing default exclusions to the list (persisted on Save, like the other edits).

Migration behaviour

Existing list	Result
stock (Epic+Steam+Riot)	+ Xbox, file rewritten, marker=1
already has Xbox + stock	no-op, marker=1
customized (a default removed)	untouched, marker=1
Xbox removed later	not re-added (marker already 1)
fresh install (no file)	seeded with Xbox

Daemon unchanged — it has no built-in defaults, it reads Excluded.txt.

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