Fix MLT MultiPolygon ring grouping for PostGIS-clipped geometries

[trasch]

Problem

MLT tiles showed triangular artifacts in maplibre-gl-js when rendering fill polygons at specific zoom levels (e.g. z7). The PBF (MVT) version of the same tile rendered correctly.

Root cause

PostGIS ST_ClipByBox2D can produce MultiPolygons where interior rings (holes) are stored as separate single-ring polygons instead of being associated with their parent exterior ring.

• The MVT encoder flattens all rings into one list; the MVT decoder re-groups them using winding order heuristics (classifyRings), which accidentally fixes the broken PostGIS structure.
• The MLT encoder preserved the PostGIS structure as-is, keeping holes as separate filled polygons. maplibre rendered these holes as filled polygons, causing overlapping triangular artifacts.

Fix

In MLTEncoder.encode, before encoding a MultiPolygon:

1. Detect whether any single-ring polygon has a clockwise (interior) ring — a sign that holes were separated from their parent.
2. If so, re-group all rings using the same winding-order heuristic as the MVT decoder: non-CW rings start a new polygon, CW rings become holes of the current polygon.
3. Correctly structured MultiPolygons are left untouched.

Verification

• Tile 7/65/47 landuse_a layer: went from 2 → 26 MultiPolygons with holes, matching the PBF output exactly.
• All 57 existing MLT tests pass.
• Added 3 regression tests covering the fix.

Files changed

• Sources/MVTTools/Coders/MLT/MLTEncoder.swift — the fix
• Tests/MVTToolsTests/MLT/MLTRoundtripGeometryTests.swift — regression tests

[trasch]

Closing: the winding-order "regrouping" heuristic in this PR is actively harmful. It merges legitimate MultiPolygons that contain several separate same-winding components (e.g. 28 disjoint forest patches) into a single polygon with bogus holes, producing the very triangular artifacts it aimed to fix. The correct hole/interior-ring association is guaranteed by the data source (PostGIS ST_MakeValid(method=structure)); the MLT encoder simply passes the structure through unchanged.