Stabilize frozen drag release cursor mapping

[yvette-carlisle]

Summary

• keep rounded overlay cursor mapping for live cursor, loupe, and RGB sampling
• scope floored coordinate mapping to active frozen selection drag updates only
• cover the frozen drag cursor-move plus release path with a regression test

Testing

• cargo make fmt-rust
• cargo test -p rsnap-overlay overlay_window_event_global_position_rounds_fractional_scaled_positions --lib
• cargo test -p rsnap-overlay frozen_selection_drag_uses_non_rounding_cursor_move_updates_without_shifting_cursor_context --lib
• cargo test -p rsnap-overlay frozen_selection_drag_ --lib
• cargo make test-rust

Refs XY-237

[yvette-carlisle]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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