feat: add a submarine cable tool

Add find_submarine_cables over the bundled TeleGeography data: cables
passing through or landing in an area, or matching a name, with landing
points and the dataset's attribution. The local MCP server's fetch reads
the bundled data files a browser would load by URL, and nothing outside
that directory.

server/mcp/services.js

@@ -3,11 +3,20 @@
  * running God's Eye View server's `/api` routes.
  */
 
+import { readFile } from 'node:fs/promises';
+import { fileURLToPath } from 'node:url';
 import { createToolServices } from '../../src/tools/services.js';
 
+// Bundled data some sources read by module-relative URL.
+const BUNDLED_DATA = new URL('../../src/data/local_data/', import.meta.url);
+
 export const DEFAULT_API_BASE = 'http://localhost:4173';
 
-/** Resolve the sources' relative `/api/...` requests against `apiBase`. */
+/**
+ * Resolve the sources' relative `/api/...` requests against `apiBase`, and
+ * serve `file:` URLs inside the bundled data directory from disk, as a
+ * browser would load those module-relative assets.
+ */
 export function createApiFetch({
   apiBase = DEFAULT_API_BASE,
   fetchImpl = (...args) => globalThis.fetch(...args),
@@ -15,13 +24,34 @@ export function createApiFetch({
   const base = new URL(apiBase);
   if (!['http:', 'https:'].includes(base.protocol))
     throw new TypeError(`apiBase must be an http(s) URL: ${apiBase}`);
-  return (input, init) =>
-    fetchImpl(
+  return (input, init) => {
+    const url = String(input instanceof Request ? input.url : input);
+    if (url.startsWith('file:')) return readBundled(url);
+    return fetchImpl(
       typeof input === 'string' && input.startsWith('/')
         ? new URL(input, base)
         : input,
       init,
     );
+  };
+}
+
+async function readBundled(url) {
+  const href = new URL(url).href;
+  if (!href.startsWith(BUNDLED_DATA.href) || href.includes('/../'))
+    return new Response(null, { status: 404 });
+  try {
+    const body = await readFile(fileURLToPath(href));
+    return new Response(body, {
+      headers: {
+        'Content-Type': href.endsWith('.json')
+          ? 'application/json'
+          : 'application/octet-stream',
+      },
+    });
+  } catch {
+    return new Response(null, { status: 404 });
+  }
 }
 
 /** Construct every service Core's tools read, backed by the local server. */