Show build progress instead of a static "Building…"

terrain.pmtiles can take minutes (tiles fetched one at a time) with no
way to tell a slow build from a stuck one. Now reports tiles_done/
tiles_total into the same status Hash the frontend already polls, and
the panel shows elapsed time (both files) plus tile counts (terrain
only) so the numbers visibly move every ~3s poll instead of sitting on
one static string.

lib/maptool/pmtiles_builder.rb

@@ -66,10 +66,14 @@ module Maptool
 
     # fetch_tile: same injectable-seam reasoning as fetch_build_url above,
     # for the many individual Terrarium tile fetches this needs instead of
-    # one index lookup.
+    # one index lookup. This build can run for minutes with nothing else to
+    # show for it, so write_terrarium_mbtiles reports tiles_done/tiles_total
+    # into the status Hash as it goes — otherwise a poller sees "running"
+    # the whole time with no way to tell it apart from stuck.
     def self.build_terrain(bounds, data_dir:, fetch_tile: method(:fetch_terrarium_tile))
       begin_run!(TERRAIN_MUTEX, TERRAIN_STATUS)
-      Thread.new { run_job(TERRAIN_MUTEX, TERRAIN_STATUS) { build_terrain_pmtiles(bounds, data_dir, fetch_tile) } }
+      progress = ->(done, total) { TERRAIN_MUTEX.synchronize { TERRAIN_STATUS.merge!(tiles_done: done, tiles_total: total) } }
+      Thread.new { run_job(TERRAIN_MUTEX, TERRAIN_STATUS) { build_terrain_pmtiles(bounds, data_dir, fetch_tile, progress) } }
     end
 
     def self.begin_run!(mutex, status)
@@ -117,12 +121,12 @@ module Maptool
 
     # --- terrain.pmtiles ---
 
-    def self.build_terrain_pmtiles(bounds, data_dir, fetch_tile)
+    def self.build_terrain_pmtiles(bounds, data_dir, fetch_tile, progress)
       south, west, north, east = bounds
       tmp_mbtiles = File.join(Dir.tmpdir, "maptool-terrain-#{Process.pid}-#{Time.now.to_i}.mbtiles")
       tmp_path = File.join(data_dir, "terrain.pmtiles.tmp")
 
-      write_terrarium_mbtiles(tmp_mbtiles, south, west, north, east, fetch_tile)
+      write_terrarium_mbtiles(tmp_mbtiles, south, west, north, east, fetch_tile, progress)
       run_pmtiles!("convert", tmp_mbtiles, tmp_path)
       File.rename(tmp_path, File.join(data_dir, "terrain.pmtiles"))
     ensure
@@ -141,7 +145,7 @@ module Maptool
     # park-sized region, slow for a big one. Add a small bounded-concurrency
     # fetch pool if regenerating a large area turns out to matter in
     # practice.
-    def self.write_terrarium_mbtiles(path, south, west, north, east, fetch_tile)
+    def self.write_terrarium_mbtiles(path, south, west, north, east, fetch_tile, progress)
       db = SQLite3::Database.new(path)
       db.execute_batch(<<~SQL)
         CREATE TABLE metadata (name TEXT, value TEXT);
@@ -152,9 +156,19 @@ module Maptool
         db.execute("INSERT INTO metadata (name, value) VALUES (?, ?)", [name, value])
       end
 
-      (0..TERRAIN_MAX_ZOOM).each do |z|
-        tile_range(z, south, west, north, east).each do |x, y|
+      # tile_range per zoom computed once upfront (cheap — no fetching, just
+      # tile math) and reused below, so progress can report a real total
+      # from the very first call instead of only once the run finishes.
+      by_zoom = (0..TERRAIN_MAX_ZOOM).map { |z| [z, tile_range(z, south, west, north, east)] }
+      total = by_zoom.sum { |_z, tiles| tiles.size }
+      done = 0
+      progress.call(done, total)
+
+      by_zoom.each do |z, tiles|
+        tiles.each do |x, y|
           bytes = fetch_tile.call(z, x, y)
+          done += 1
+          progress.call(done, total)
           next unless bytes
 
           # MBTiles uses TMS row numbering (0 at the south) — the opposite