FocusTrapScope class

Standard "modal focus trap" for an in-tree overlay panel — wraps child (the modal's own content) in a FocusScope that autofocuses as soon as this widget mounts (i.e. exactly when the overlay it belongs to appears, matching this package's "always mounted, panel nullable" convention — NeonDialog.overlaySlot, PauseOverlay), and restores focus to whatever had it right BEFORE this widget mounted, the moment it's removed from the tree again (the overlay closing).

FEAT-82 closes exactly the gap PauseOverlay's own doc comment (FEAT-53) flagged: a bare FocusScope(autofocus: true) grabs focus when a modal appears but has no way to give it back to the right place when the modal's subtree gets torn down — Flutter's own FocusManager just falls back toward the tree root instead, so a keyboard/gamepad player closing a dialog would otherwise lose their place entirely.

Deterministic order: traversal WITHIN child uses Flutter's own default FocusTraversalPolicy (tree/paint order) — this widget adds no custom ordering, since none of the trap/release behavior it exists for depends on child order.

Purely additive for touch-only play: nothing here changes what a tap does; the whole effect is scoped to keyboard/gamepad focus, which a touch-only session never engages with (same guarantee PressableScale's own focus-ring doc comment gives for the same reason).