Migrate polylabel vendor from mapbox to mapnik fork

[Copilot]

mapbox/polylabel deleted its C++ implementation in mapbox/polylabel#124. mapnik/polylabel is the maintained fork that restores and improves the C++ port (v2.1.0 algorithmic improvements: clamped cell sizes, flattened coordinate buffers, block-based bbox skipping, aggressive queue pruning).

Changes

• .gitmodules — point submodule URL to https://github.com/mapnik/polylabel.git
• vendor/polylabel — update submodule ref from 6ed5f1a (mapbox 1.1.0) to 1edc167 (mapnik latest)
• vendor/polylabel.cmake — update INTERFACE_MAPLIBRE_URL and INTERFACE_MAPLIBRE_AUTHOR to reflect the new fork

No source changes needed — the fork preserves the same include path (<mapbox/polylabel.hpp>), namespace (mapbox::polylabel), and function signature, adding only an optional distance output parameter.

• Fixes Migrate to mapnik/polylabel #4407

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0% +47.7Ki  +0.0% +14.9Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-4426-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +55% +63.3Mi  +472% +28.2Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-4426-compared-to-legacy.txt

[github-actions]

Benchmark Results ⚡

Benchmark                                                          Time             CPU      Time Old      Time New       CPU Old       CPU New
-----------------------------------------------------------------------------------------------------------------------------------------------
OVERALL_GEOMEAN                                                 +0.0056         +0.0053             0             0             0             0

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/benchmark-results/pr-4426-compared-to-main.txt