fix(tiles): derive the tile origin from the configured TileJSON (#933)

`createVectorTileSource` took `tileJsonUrl` and `allowedOrigin` as independent
options, and `createOpenFreeMapSource` hardcoded both to the default host. A
caller that repointed only `tileJsonUrl` — the obvious single change — kept the
default `allowedOrigin`, so the TileJSON's absolute upstream `tiles[0]` passed
the origin check and every tile was fetched from the default host instead. No
error, no warning, and nothing reached the configured mirror.

`allowedOrigin` now defaults to the origin of `tileJsonUrl`, so repointing one
option is enough, and a mirror that serves the default host's TileJSON verbatim
fails with the existing `Invalid vector tile origin` instead of silently
bypassing itself. The origin check is unchanged: a TileJSON still cannot point
the client at a host the caller did not configure.

All three existing callers pass `allowedOrigin` explicitly
(`openFreeMap.js`, `alpr/source.js`, `traffic/flowSource.js`), and an explicit
value still overrides the derived one, so behaviour is unchanged for every
current call site.

src/sources/vectorTiles.test.mjs

@@ -203,3 +203,45 @@ test('polygon-selected tiles keep nearest-first order and cached revisits issue
     'over-budget selection is rejected before fetching',
   );
 });
+
+test('a configured tileJsonUrl sets the tile origin, and a foreign tiles[0] is still refused', async () => {
+  // Without an explicit allowedOrigin the mirror's own TileJSON is honoured…
+  const mirrored = createVectorTileSource({
+    tileJsonUrl: 'http://127.0.0.1:12348/planet',
+    decode: () => [],
+    fetchImpl: async () =>
+      Response.json({ tiles: ['http://127.0.0.1:12348/p/{z}/{x}/{y}.pbf'] }),
+  });
+  assert.equal(
+    (await mirrored.getMetadata()).template,
+    'http://127.0.0.1:12348/p/{z}/{x}/{y}.pbf',
+  );
+
+  // …while a TileJSON that names a different host is refused rather than
+  // silently fetched, which is what the origin check is for.
+  const foreign = createVectorTileSource({
+    tileJsonUrl: 'http://127.0.0.1:12348/planet',
+    decode: () => [],
+    fetchImpl: async () =>
+      Response.json({
+        tiles: ['https://tiles.example/planet/{z}/{x}/{y}.pbf'],
+      }),
+  });
+  await assert.rejects(foreign.getMetadata(), { retryable: false });
+});
+
+test('an explicit allowedOrigin still wins over the one derived from tileJsonUrl', async () => {
+  const source = createVectorTileSource({
+    tileJsonUrl: 'http://127.0.0.1:12348/planet',
+    allowedOrigin: 'https://tiles.example',
+    decode: () => [],
+    fetchImpl: async () =>
+      Response.json({
+        tiles: ['https://tiles.example/planet/{z}/{x}/{y}.pbf'],
+      }),
+  });
+  assert.equal(
+    (await source.getMetadata()).template,
+    'https://tiles.example/planet/{z}/{x}/{y}.pbf',
+  );
+});