Overlay: windows that can draw position-aware widgets (compass, rings, markers) #184

Goal

Today's overlay (IOverlay, OverlayContentBuilder, Windows-only OverlayWindow) renders text lines. Position features need small custom-drawn widgets: a compass strip, distance rings, a target marker, a map pane.

Scope

• Extend the overlay abstraction so a feature can contribute a drawable widget with its own size and update rate, composited into the existing click-through, topmost, borderless window (or a sibling window), driven by the same UI tick.
• Keep it behind an interface with a no-op fallback on non-Windows (AGENTS.md rule 4); document what each platform can do.
• Per-widget enable, screen position and scale in Settings; remember positions; hide the overlay when Elite is not the foreground app. NOTE: the current overlay does not do this (it has no foreground-window detection and is always topmost); see Overlay: topmost self-resizing window can minimize/steal focus from the game and EDCopilot on system jump #204.
• All colours from Themes/Theme.axaml; no hardcoded values.

Acceptance criteria

• A sample widget (e.g. a heading compass fed by the position model) renders over the game and updates smoothly.
• Widgets can be positioned, scaled and turned off individually and the choice persists.
• Non-Windows builds compile and degrade to the dashboard without errors.
• Measured cost with the overlay idle is negligible.

Depends on the surface position model for the first real widget.

Part of the epic #182.

Clean-room: see the rule on the epic. No SrvSurvey code, assets, maps or data.

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