[neovolgan]

Importer side of contract section 4 (#2). This PR is stacked on #4 (fence layer), which is stacked on #3.

What it does

• barrier=fence|wall|city_wall|retaining_wall lines and closed ways from GeoJSON or OSM become FenceLines.
• Shape: the projected path is walked along cell edges by FenceStaircase. Straight runs stay single segments. A diagonal becomes a staircase that follows it (the test checks it stays within 1.5 tiles), and a step straight back cancels the previous one.
• Look: pz:fence picks a style that exists in the map. Otherwise fence_type picks one of four vanilla looks: sheet, planks, picket or wire. Only the defaults actually used are added to FenceStyles. pz:posts sets the post spacing.
• Gates:
  • pz:gates (at in metres along the source line, opens left/right, style) places a gate on the edge nearest that point;
  • barrier=gate nodes within a tile of a fence become a vanilla wooden wicket on the nearest edge;
  • gates that cannot be placed are dropped and counted in the import notes.
• Source: the real lon/lat path, source ID and tags are kept.
• Preserve edits: authored fences stay and sourced fences are refreshed, not duplicated.
• Limits: lines outside the map or beyond the limits are counted in the import summary note.

Tests: a new case in FenceLineTests covers:

• staircase shape;
• styles from fence_type and defaults (including an unknown pz:fence);
• the post spacing;
• pz:gates and a barrier=gate node landing on the right edges;
• the vanilla wicket drawn by the composer;
• the kept source path;
• reimport with Preserve edits.

Results: core 344/344 passed, npm test 184/184 passed.

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