Preserve Retina density in screenshot exports

[yvette-carlisle]

Summary

• Keep Retina screenshot exports at full backing-pixel resolution while writing PNG pHYs density from the capture screen scale.
• Add FFI and Swift export overloads for screen-scale-aware PNG encoding, with probe/header coverage.
• Make the selection size badge explicit about output pixels and HiDPI scale, e.g. 240x160 px @2x.

Verification

• cargo make fmt-check
• cargo make check-rust
• cargo make check-swift
• cargo make check-vstyle
• cargo make test-host-reset
• cargo test -p rsnap-overlay selection_size_badge_text_uses_monitor_pixel_dimensions
• cargo test -p rsnap-overlay selection_size_badge_layout_keeps_visual_bounds_inside_badge_rect
• git diff --check
• scripts/smoke/macos.sh (PASS: repeated drag freeze, click guard, HUD, loupe)
• scripts/perf/local.sh (PASS; export PNG 1440x900: 33.054ms / 900ms budget)