Symbol Placement Style Options Line start and Line end

[JoshuaJMoore]

line-start / line-end symbol placement

Adds two new values for symbol-placement:
line-start and line-end, which place a single symbol at the first or last point of a line geometry respectively.

How it works
Anchor selection:
getStartAnchor uses line[0], getEndAnchor uses line[last], with the angle oriented along the line direction.
Text offset shifting:
Glyph offsets are shifted so they all extend inward along the line (positive for line-start, negative for line-end), since glyphs can only be laid out in one direction from an endpoint.
No line clipping:
Unlike line placement which clips geometry to the tile boundary (creating artificial endpoints), line-start/line-end operate on the raw feature geometry to preserve true endpoints.
Vector tile deduplication:
Lines in vector tiles include buffer geometry that extends into neighboring tiles. To prevent duplicate icons, anchors outside the tile's [0, EXTENT) coordinate range are rejected. Since only the tile that owns the true start/end point will have it within bounds, exactly one icon is rendered per line endpoint, no cross-tile coordination needed.

Example Image:
symbol-placement: line start for ferry routes using data from mapbox://styles/mapbox/streets-v12

Launch Checklist

• Make sure the PR title is descriptive and preferably reflects the change from the user's perspective.
• Add additional detail and context in the PR description (with screenshots/videos if there are visual changes).
• Manually test the debug page.
• Write tests for all new functionality and make sure the CI checks pass.
• Document any changes to public APIs.
• Post benchmark scores if the change could affect performance.
• Tag @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes.
• Tag @mapbox/gl-native if this PR includes shader changes or needs a native port.
• Tag @mapbox/gl-native if this PR disables any test because it also needs to be disabled on their side.
• Create a ticket for gl-native to groom in the MAPSNAT JIRA queue if this PR includes shader changes or features not present in the native side or if it disables a test that's not disabled there.

[github-actions]

Hey, @JoshuaJMoore 👋 Thanks for your contribution to Mapbox GL JS!

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[JoshuaJMoore]

@mapbox/map-design-team @mapbox/static-apis

[JoshuaJMoore]

Thoughts on this one? @ibesora