[brawer]

Makes coarse zooms of both tile sets show the most important places, using the importance column from #853. Part of #713.

What changes

• The conflated.pmtiles overview (z0–12) gets an importance property and the edit suggestions in diffed-places.pmtiles get @importance. Both are rounded to two decimals, so a tile holds at most 101 distinct values.
• A tippecanoe feature filter (-j) on that property decides what each coarse zoom shows. z0 shows the top 1% (>= 0.99), z1 4% (>= 0.96), z2 16% (>= 0.84) and z3 64% (>= 0.36). From z4 on, everything is shown. The share grows four times per zoom because each tile splits into four, so features per tile stay about the same.
• Random dot-dropping is off for these two builds (-r 1 instead of -r 1.2), so importance alone decides what coarse zooms show. --drop-densest-as-needed remains the safety net for oversized tiles.
• The detail pass (z13–16) is unchanged.

A per-feature "tippecanoe": {"minzoom": N} would have been the obvious approach. With tippecanoe v2.49, though, it made tippecanoe drop nearly all features at every zoom: in a Swiss test, 2 of 1,436 survived at z0. I didn't find the root cause, so this PR uses the filter.

Swiss test run

Features per zoom in the conflated overview:

	z0	z1	z2	z3	z4	z6	z12
main	1,436	1,724	2,069	2,484	2,983	4,332	14,941
this PR	314	934	3,393	12,690	19,663	19,740	23,438

The lowest importance shown at z0 is 0.99. The top spiders at z0 are stadt_zuerich_ch, starbucks_eu, soeder_ch, mcdonalds and rotpunkt_apotheken_ch. In diffed-places.pmtiles, z0 shows 67 features and z2 770, with the lowest at z2 being 0.84.

Note that z4–z12 now hold more features than before, since dot-dropping is off. On the planet, --drop-densest-as-needed will thin out dense tiles there.

Tests

cargo test and cargo clippy --all-targets pass. New unit tests cover the filter, the thresholds, rounding, and both GeoJSON writers. The integration test checks @importance on edit features.

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