fix(overpass): keyless gate, loopback admission headroom and per-phase arrival budgets

Address gekh's review of 69a87fe2:

- P1: /api/overpass now uses admitSameSiteKeyless, like /api/terrain/heights,
  so an instance behind the README's reverse proxy keeps its Overpass-backed
  layers; cross-site shapes are still refused and forwarding headers no
  longer launder them. SECURITY.md, the gate header and CURRENT-STATE say so.
- P2: admission keys IPv6 peers by /64 and folds IPv4-mapped peers
  (clientNetworkKey), and loopback has its own 300/min window outside the
  900/min global, so a few LAN peers cannot lock the owner's browser out.
- P2 (track): the settle and reveal frames each get their own 500 ms budget,
  measured from moveEnd and from the settle frame respectively, so a ~300 ms
  SwiftShader frame no longer trips the reveal deadline while both stay
  inside the 1 s idle refresh.
- Smaller: drop the dead settings-block check and stale probe comment in
  query.js, clamp whitespace-tolerant [ timeout : N ], and add tests for the
  global window, the /64 keying, loopback headroom, proxied admission and
  cache hits spending admission but not upstream budget.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

server/providers/common/rateLimitKeys.test.mjs

@@ -0,0 +1,37 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { clientNetworkKey, isLoopbackPeer } from './rate-limit.js';
+
+const req = (remoteAddress) => ({ socket: { remoteAddress } });
+
+test('clientNetworkKey keys IPv6 peers by /64 and folds IPv4-mapped peers', () => {
+  assert.equal(clientNetworkKey(req('2001:db8:1:2::1')), '2001:db8:1:2::/64');
+  assert.equal(
+    clientNetworkKey(req('2001:0db8:0001:0002:abcd:ef01:2345:6789')),
+    '2001:db8:1:2::/64',
+  );
+  assert.equal(clientNetworkKey(req('2001:DB8::1')), '2001:db8:0:0::/64');
+  assert.equal(clientNetworkKey(req('fe80::1%en0')), 'fe80:0:0:0::/64');
+  assert.equal(clientNetworkKey(req('::1')), '0:0:0:0::/64');
+  assert.equal(clientNetworkKey(req('::ffff:10.0.0.5')), '10.0.0.5');
+  assert.equal(clientNetworkKey(req('10.0.0.5')), '10.0.0.5');
+  assert.equal(clientNetworkKey(req(undefined)), 'local');
+  assert.equal(clientNetworkKey(req('not-an-address')), 'not-an-address');
+});
+
+test('isLoopbackPeer recognizes every loopback form and nothing else', () => {
+  for (const address of ['127.0.0.1', '127.9.9.9', '::1', '::ffff:127.0.0.1']) {
+    assert.equal(isLoopbackPeer(address), true, address);
+  }
+  for (const address of [
+    '10.0.0.1',
+    '::ffff:10.0.0.1',
+    '2001:db8::1',
+    '::2',
+    'local',
+    '',
+    undefined,
+  ]) {
+    assert.equal(isLoopbackPeer(address), false, String(address));
+  }
+});