[pmeems]

Phase 2 of the Map Model and Basic Navigation plan. Stacked on #34.

Motivation

Multi-layer rendering (Phase 3) and navigation (Phases 4 and 5) need one reusable map-to-screen transformation in Core. Today the only transformation is activex::ViewTransform, which lives in the OCX and is rebuilt from the dataset extent on every draw.

What changes

• mwg::Viewport (mapwingis/map/Viewport.h) holds a pixel size plus a view (center and map units per pixel). It provides MapToScreen, ScreenToMap, VisibleExtent, FitExtent(extent, padding), SetView, and SetSize.
• MapPoint, ScreenPoint, PixelPoint and ToPixel, the only place where screen coordinates are rounded.
• ViewportTests.cpp with 36 tests. No window and no ActiveX.
• A "Viewport" section in docs/ARCHITECTURE.md.

Key design decisions

• State is center + resolution, not an extent. One resolution applies to both axes, so the view never distorts geometry. Resizing keeps the center and scale, which Phase 3 resizing and Phase 4 zoom/pan need.
• Continuous screen coordinates. Pixel (i, j) covers [i, i+1) x [j, j+1), and Y points down. FitExtent maps the extent edges to the centers of the outermost pixels, so rounding never pushes boundary geometry out of the viewport.
• Invalid input: API misuse (negative size, non-finite view, negative padding) throws InvalidArgument. Non-finite points or overflowing results return std::nullopt. Invalid extents and zero-sized viewports make FitExtent return false without changing the view. A zero-sized viewport keeps its view.
• Limits: the center is clamped to ±1e12, resolution to [1e-10 × max(1, |center|), 1e9], and size to 65536 px. Because the minimum resolution scales with the center, a pixel always spans many representable doubles.
• Value type, not PImpl. Like Extent, Viewport is small and copyable, so renderers and navigation code can take snapshots of it.
• The viewport does not reference a Map. Callers fit it to Map::GetExtent().

Dependencies / follow-up

• Based on phase-1-map-model-and-layer-collection (MapWinGIS6 — Map Model and Basic Navigation #34).
• ActiveX keeps ViewTransform for now. Replacing it with Viewport and choosing the owner of the map/viewport pair is Phase 3.

Build: Debug|x64, 0 warnings. Tests: 226 passed, 3 skipped (opt-in suites).