Visibility model: ownership, faceVisibility, presence

[jordanwallwork]

Parent: Issue #98

Implement zone ownership and visibility. Zones have owners; cards have two visibility axes.

• computeViewState(state, viewerSeat | 'omniscient') → MaskedState
• Axes: faceVisibility (all|owner|others|none), presence (visible|hidden-from-non-owners)
• Card masking: non-owners see blanks, hidden piles disappear
• Solo features: seat switcher, omniscient mode
• No per-card overrides; reveal = move to visible zone

Acceptance criteria

• computeViewState() masks by visibility rules
• Seat switcher updates view without modifying state
• Omniscient mode shows all cards
• Solo freeform play still works
• Tests verify masking for 2/4/8 player configs

Blocked by: None

[jordanwallwork]

Implemented in commit 149947b on feature/game-setup: pure computeViewState(state, viewer, visibilityMap) → MaskedState (faceVisibility + presence masking, omniscient mode) and viewController.svelte.ts seat switcher. Included in PR #125.

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