Add opt-in floating nametags above named/categorized OSM elements

[MJRobillard]

Summary

• Adds a new --nametags flag (Java only, off by default) that floats a minecraft:text_display label above every OSM element with a name (or a synthesized <housenumber> <street> address) and/or a recognized OSM category tag (amenity, shop, leisure, tourism, railway, highway, building, office, craft, landuse, natural, man_made).
• Purely additive and self-contained in a new src/nametags.rs module — no changes to existing block/entity APIs, no interaction with any other feature.
• Capped at 3000 labeled elements per world; named elements are prioritized over category-only ones when the cap applies.

Test plan

• cargo test --release --no-default-features nametags — 6 new unit tests pass (label collection, address fallback, boolean-tag skipping, way centroid, entity placement)
• cargo build --release --no-default-features — clean build, no new warnings
• cargo fmt --check / cargo clippy — clean
• Verified in a real generated world (1-mile-radius area around a Philadelphia address) that nametags render correctly alongside other features

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