Compare two map layers, and two dates, side by side

[sandesh-sp]

Purpose

Adds a two-sided comparison mode to the map so a user can hold two layer sets — or the same layers at two different dates — against each other in one viewport.

Two layouts share a single draggable divider:

• swipe — one camera, one basemap. Both sides draw the same view and the divider wipes between them, so a place is seen under one layer or the other.
• sideBySide — two cameras locked to the same centre and zoom, each with its own basemap, in panes that meet at the divider without overlapping. A place is seen under both layers at once, once per pane.

Either side can be pinned to its own ISO instant. MapComparison turns that instant into per-layer prop overrides and hands them to the engine; the engine applies what it is given to that side's layer clones and derives nothing itself. A side left unpinned follows the global timeline like any other layer.

Everything is driven through the mmgisAPI event bus — no plugin code is hardcoded in the core, and the engine surface is optional methods on IMapEngine, so an engine that does not implement them degrades to a warning rather than a crash.

Proposed Changes

• [ADD] MapComparison controller — owns the divider DOM, the layout, the divider position, and each side's layers/date.
• [ADD] comparisonTimePins.ts — turns a per-side date into the layer prop overrides that redraw that side at that instant.
• [ADD] sideBySide layout in DeckGLAdapter: a second deck instance with its own basemap, cameras locked to the same centre/zoom, panes clipped to meet at the divider.
• [ADD] mmgisAPI requests map:comparison:enable / disable / setLeftSide / setRightSide / setLayout / setDividerPosition / getState, and events map:comparison:dividerMoved / layoutChanged.
• [ADD] setComparisonLayout / getComparisonLayout and the layout, leftLayerProps, rightLayerProps fields on ComparisonConfig in IMapEngine.
• [ADD] Bearing and pitch accessors on DeckGLAdapter, needed to keep the two side-by-side cameras in lockstep.
• [ADD] 6 unit spec files covering the layout switch, date pinning, divider behaviour, and the deck.gl prop/side-by-side plumbing.
• [CHANGE] vitest.config.ts aliases PDFViewer to a stub — it holds JSX in a .js file, which Vite's default .js loader will not parse, and anything reaching Layers_ pulls it in transitively.

Issues

• None linked.

Testing

Full unit suite on macOS (Darwin 25.5.0), Node 20:

$ npx vitest run
 Test Files  98 passed (98)
      Tests  1472 passed (1472)
   Duration  11.36s

Of those, 6 files / ~90 tests are new here and cover this feature directly: mapComparisonLayout, mapComparisonDates, comparisonTimePins, deckGLSideBySide, deckGLComparisonProps.

Still worth a live smoke test on a real mission config before merge — dragging the divider in both layouts, and confirming a date-pinned side redraws against tiles for that date.