fix: include Finnish trunk roads from zoom 3

[pekkine]

Implements the per-country minzoom from #221, for Finland. Adds a pm:country=FI rule to highwayZoomsIndex next to the US ones, giving highway=trunk a minzoom of 3.

Finland tags its national road network highway=trunk, not motorway, so z3 to z5 carry almost none of it today. All of Finland fits on a phone screen at about z3, and that is the view where the road network is what lets a reader locate anything.

Refs #221

Notes

Only the geometry zoom moves. trunk_link stays at 6, kind stays major_road, and nothing changes in the default styles.

@nvkelso raised in #221 that differing thresholds make roads start and stop at country boundaries. That happens here: the Finnish network ends at the Swedish and Norwegian borders, since those countries have no rule. The same rule can be added for them if that is wanted.

Short stretches right at the border also stay at the old zoom, since country assignment is per way and a way that is not fully inside the country polygon gets none. That is a small loss, since the border itself is drawn at these zooms and already gives the reader something to locate against.

Also adds a Finland bounding box to the LayerTest country coder fixture, without which no pm:country=FI rule can be tested.

AI Assistance

This PR was created with the help of Claude Code (Claude Opus 5). All code changes have been reviewed and understood by a human contributor.

[sonarqubecloud]

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B Maintainability Rating on New Code (required ≥ A)

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