add_h3t_source() for tiled H3 data (#199)

[walkerke]

Lands @bbest's #199 (H3 tiled source), rebased onto the current 0.5 main with conflicts resolved.

Adds add_h3t_source() — an R wrapper for the h3tiles:// MapLibre protocol bundled in h3j-h3t — so H3 hex layers can be driven from a tiled {z}/{x}/{y} endpoint (one request per viewport) instead of loading a single monolithic H3J file. Includes Ben's five upstreamed h3j-h3t fixes (0.9.2 → 0.9.7) for MapLibre v3–v5, query-string tile URLs, empty tiles, and multiple sources per page / per compare side.

Conflict resolution notes

The original PR predated the 0.5 slider/flowmap/compare work and conflicted in maplibregl.js and maplibregl_compare.js:

• maplibregl.js — kept main's filter-registry set_filter (so the slider/legend filter composition is preserved) and layered Ben's new add_h3t_sources / add_h3j_sources proxy handlers alongside it.
• maplibregl_compare.js — main has since added getLayerControlVisibility() / setLayerControlVisibility() helpers that already perform Ben's getLayer() pre-check (silencing the missing-layer error event) and support multi-map compare (sync more than 2 maps  #204) and flowmap layers. Those supersede Ben's inline version, so the helpers were kept. Ben's h3t init + proxy handlers are retained.
• Removed a stray debug build-marker / console.log that survived from the PR's development commits.
• add_h3t_source() now ends with return(map) to match the package convention.

Ben's authorship is preserved on the commit. R CMD check clean (0 errors / 0 warnings / 0 notes); both JS bindings pass node --check. Supersedes #199.

Note: Mapbox parity (addH3TSource on mapboxgl.js) is still out of scope, as in the original PR.

[bbest]

Bravo! Thank you so much for your detailed review and merge :)