Add in-app regeneration of offline.pmtiles and terrain.pmtiles

A logged-in user can now trigger, from a new "Offline data" sidebar
panel, what was previously a fully manual go-pmtiles workflow
(docs/development.adoc): regenerate either file for the current map
viewport, no area-picker (same "visible area is the area" convention
the Overpass POI plugin already uses).

- lib/maptool/pmtiles_builder.rb: two independent background jobs
  (Thread + Mutex-guarded status Hash, same shape as app.rb's own GNSS
  poller), each writing to a .tmp path and atomically renaming into
  place so a build never serves a partially-written file mid-run.
  offline.pmtiles is a single `pmtiles extract` against Protomaps'
  public daily basemap build (byte-range extraction, not a full
  ~120GB download). terrain.pmtiles has no equivalent public
  Terrarium-encoded archive to extract from, so it fetches individual
  Terrarium tiles itself, packs them into an intermediate MBTiles
  (new sqlite3 gem), then the same `pmtiles convert`.
- 4 new app.rb routes (POST .../build, GET .../build-status) plus
  size/mtime added to the existing availability endpoints.
- public/js/offline_data.js: new sidebar panel, polling build-status
  the same self-rescheduling-setTimeout shape livetrack.js already
  uses for its own refresh loop.
- Both network-heavy pieces (the Protomaps build index, per-tile
  fetches) are injectable, so test/test_pmtiles_builder.rb exercises
  the actual build pipeline against a fake `pmtiles` binary without
  hitting a real network, matching this suite's existing
  keep-it-offline convention for external services.

Gemfile.lock

@@ -95,6 +95,16 @@ GEM
     snaky_hash (2.0.7)
       hashie (>= 0.1.0, < 6)
       version_gem (~> 1.1, >= 1.1.14)
+    sqlite3 (2.9.6-aarch64-linux-gnu)
+    sqlite3 (2.9.6-aarch64-linux-musl)
+    sqlite3 (2.9.6-arm-linux-gnu)
+    sqlite3 (2.9.6-arm-linux-musl)
+    sqlite3 (2.9.6-arm64-darwin)
+    sqlite3 (2.9.6-x86-linux-gnu)
+    sqlite3 (2.9.6-x86-linux-musl)
+    sqlite3 (2.9.6-x86_64-darwin)
+    sqlite3 (2.9.6-x86_64-linux-gnu)
+    sqlite3 (2.9.6-x86_64-linux-musl)
     tilt (2.9.0)
     uri (1.1.1)
     version_gem (1.1.15)
@@ -122,6 +132,7 @@ DEPENDENCIES
   rubyzip (~> 2.4)
   sinatra (~> 4.2)
   sinatra-contrib (~> 4.2)
+  sqlite3 (~> 2.0)
 
 CHECKSUMS
   anonymous_loader (0.1.3) sha256=084a18e2439144d955447dc11dfc982f41fcd1583ad32d4d55151325dc44cb55
@@ -171,6 +182,16 @@ CHECKSUMS
   sinatra (4.2.1) sha256=b7aeb9b11d046b552972ade834f1f9be98b185fa8444480688e3627625377080
   sinatra-contrib (4.2.1) sha256=10d091c944d268aa910c618ea40a3c3ebe0533e6e32990d84af92235a3d26b4a
   snaky_hash (2.0.7) sha256=7d02c70012a3f932e48860cd024577908300c9aa615e0cb9b450aaa749cbcb4d
+  sqlite3 (2.9.6-aarch64-linux-gnu) sha256=d8b1f7d23efd7abac285775a9566562fc7debfef79d594e3a20354406fb7907c
+  sqlite3 (2.9.6-aarch64-linux-musl) sha256=3579e1c98cdc7ff5c3722847bb63ed4e1efb7ff675cb5e1e48ef2d4da5fb3bc9
+  sqlite3 (2.9.6-arm-linux-gnu) sha256=33541500e3615da02afe54a9cc38b17a6985d3cf9d8b76d6d0a83002f114e7ec
+  sqlite3 (2.9.6-arm-linux-musl) sha256=c5490af48bb228fefa54314e9541375c3907e70f8109f3881b5ff97e1c93ae33
+  sqlite3 (2.9.6-arm64-darwin) sha256=849b5d7f795e60fe25076d62c72dd722beb45b3850b516ad978d60ee848ec15b
+  sqlite3 (2.9.6-x86-linux-gnu) sha256=fbaa9f46f9708f57dd8a459b37fc269f9613e0cacf1547df01aa439cc45c20c0
+  sqlite3 (2.9.6-x86-linux-musl) sha256=6715026fbb5530e810b28ef43b9c4f84cd3c991f67b9d808a31fcc32b847abbd
+  sqlite3 (2.9.6-x86_64-darwin) sha256=b5842fea77781c14da03fa7bc0feb82db03a69e135affcb6f5399cbd2797a5f3
+  sqlite3 (2.9.6-x86_64-linux-gnu) sha256=613188ce02f614126ddbc38c5e217ccffd6306d0dcd9adca9764547aa890a634
+  sqlite3 (2.9.6-x86_64-linux-musl) sha256=d493b11818a3573387a1d56e1ee8fa00da23a683a7a1cc063e7a0feeed843abf
   tilt (2.9.0) sha256=da5735d0280bba96e9a91041bb14aee435ccad5c17b0fa519249ae543d9aa3a5
   uri (1.1.1) sha256=379fa58d27ffb1387eaada68c749d1426738bd0f654d812fcc07e7568f5c57c6
   version_gem (1.1.15) sha256=a73241587b29252e3567e0c818ded80730eb754d43b508f6ce4db22ca9ba27d0