Truncate noisy build-failure output, fix the recurring terrain death

The web UI was showing a raw ~500k-character stdout/stderr dump on a
failed build — go-pmtiles' extract logs one progress line per chunk,
which is fine on a terminal (it overwrites itself) but not captured
non-interactively. command_output now keeps just the tail, and the
status line renders it with preserved line breaks in a scrollable box
instead of one unreadable blob.

Root cause of the actual failure (terrain build dying ~11 minutes in
with an HTTP/2 stream reset): go-pmtiles' default --overfetch=0.05
merged this multi-country planet.pmtiles extract into a single ~60GB
request with no way to resume after a dropped stream. --overfetch=0
splits it into many smaller requests instead.

lib/maptool/pmtiles_builder.rb

@@ -513,7 +513,17 @@ module Maptool
       extracts = sources.each_with_index.map do |item, i|
         report_step.call(progress_step("extracting #{item["name"]}", labels, i))
         extract_path = File.join(scratch_dir, "extract-#{i}.pmtiles")
-        run_pmtiles!("extract", item["url"], extract_path, "--region=#{region_path}", "--maxzoom=#{TERRAIN_MAX_ZOOM}")
+        # --overfetch=0: go-pmtiles' default (0.05) merges nearby byte
+        # ranges into fewer, bigger HTTP requests to save round-trips —
+        # for planet.pmtiles (global z0-12, tens of GB) covering a
+        # multi-country selection that collapsed the whole extract into
+        # a *single* ~60GB request, which then had no way to resume after
+        # an HTTP/2 stream reset 11 minutes in (see the "died again"
+        # 2026-09-12 build). Disabling merging trades that one fragile,
+        # unresumable mega-request for many small ones spread over
+        # --download-threads (default 4) — slower on a clean run, but no
+        # longer one dropped stream away from losing 20 minutes of transfer.
+        run_pmtiles!("extract", item["url"], extract_path, "--region=#{region_path}", "--maxzoom=#{TERRAIN_MAX_ZOOM}", "--overfetch=0")
         extract_path
       end
 
@@ -621,11 +631,23 @@ module Maptool
       ENV.fetch("PLANETILER_JAR", "/usr/local/lib/planetiler.jar")
     end
 
+    # go-pmtiles' extract logs one progress line per chunk fetched
+    # ("fetching N dirs, M chunks, K requests") — fine on a terminal where
+    # it overwrites itself, but captured non-interactively that's one line
+    # per chunk, easily hundreds of thousands of characters for a big
+    # region. The actual failure is always at the end, so keep only the
+    # tail — same story for osmium/planetiler's own chatty stdout.
+    OUTPUT_TAIL_LINES = 40
+
     # Some CLI tools (go-pmtiles included) write their actual error to
     # stdout rather than stderr — combine both rather than risk raising a
     # BuildError with nothing useful in it.
     def self.command_output(stdout, stderr)
-      [stdout, stderr].map(&:strip).reject(&:empty?).join("\n")
+      lines = [stdout, stderr].map(&:strip).reject(&:empty?).join("\n").lines
+      return lines.join if lines.size <= OUTPUT_TAIL_LINES
+
+      omitted = lines.size - OUTPUT_TAIL_LINES
+      "(#{omitted} earlier lines omitted)\n#{lines.last(OUTPUT_TAIL_LINES).join}"
     end
     private_class_method :command_output