fix(portrayal): place centred area symbols per S-52 §8.5.1

[beetlebugorg]

Centred area symbols (anchorages, restricted/cable/pipeline areas) were placed by putting the S-101 catalogue pivot on the area's point. For the "…RES" fan-out symbols that pivot is authored at the SVG corner, so the glyph hung ~100px off its area: misplaced (sometimes over a building), the tap at the area missed it, and it read as the symbol "moving around".

• Anchor at the pole of inaccessibility (Mapbox polylabel), hole-aware, so the representative point stays inside concave areas and off holes (S-52 §8.5.3 representative point of an area).
• Centre the PRIMARY centred symbol's glyph on that point "so it is evident which area the symbol applies to"; additional symbols keep the catalogue offset and fan out "to prevent overwriting" (S-52 §8.5.1 — a centred symbol plus offset extras). Baked as a pivot_center tag; the client selects a glyph-centred "ctr:" sprite variant for it.

Requires a re-bake. Not yet implemented: the §8.5.1 rule "if the centre of the symbol bounding box falls outside the area it must not be drawn" (zoom-dependent in a tiled renderer; needs a client render-time test).