Explore embedding UnicodeMaps in a TUI (Tachikoma.jl) as a MapWidget

[zsoerenm]

Goal

Explore using UnicodeMaps inside a terminal-UI framework — concretely
Tachikoma.jl — and, if it fits,
provide a reusable map widget (a pannable/zoomable panel) rather than only
the standalone worldmap / explore entry points.

Why it should fit

render(center, zoom; size = (w, h)) is stateless and returns a w × h grid of
braille chars + 24-bit colors. That maps directly onto Tachikoma's
Model / update! / view architecture and its Buffer / set_char! model:

• Model: lon, lat, zoom, and a reusable TileSource (for its tile cache).
• view(model, frame): render at the widget's area size, then blit each
  cell into frame.buffer via set_char! (fg = cell color, bg = mc.background).
• update!(model, event): arrow / + / - keys mutate lon/lat/zoom
  (reuse the existing pan/zoom math).

Standalone explore() and frame() would not be used inside Tachikoma — the
framework owns the terminal, event loop and double-buffering; only the core
render() is reused.

Known caveat

Tile fetching is synchronous over HTTP. In a 60 fps loop, the first visit to
an uncached tile stalls a frame (~100–300 ms). Cached tiles are instant, so
revisiting is smooth, but exploring new areas hitches. The clean fix is async
tile loading (fetch in a Task, redraw when ready) — which would also benefit
the standalone explore().

Tasks

• Add a public embedding API so consumers don't reach into mc.canvas.grid
  — e.g. cells(mc) yielding (row, col, char, color, background).
• Export the pan/zoom helpers (pan_center) for reuse in a host update!.
• Async / background tile prefetch (fetch off the render path; redraw on arrival).
• Prototype a MapWidget Model/update!/view against Tachikoma's real API
  and add it as an example (or an optional package extension).
• Handle wide (CJK) label glyphs that can misalign a fixed-width cell grid.

References

• Standalone interactive loop for comparison: src/interactive.jl (explore).
• Renderer: src/render.jl (render, pan_center).