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/same-site.js

@@ -58,7 +58,7 @@ function forwardedProtocol(req) {
 
 /**
  * Cross-site gate for keyless routes the map needs in order to render
- * (`/api/terrain/heights`). It refuses the same cross-site browser shapes as
+ * (`/api/terrain/heights`, `/api/overpass`). It refuses the same cross-site browser shapes as
  * `admitSameSite` (a `Sec-Fetch-Site` other than same-origin/none, a foreign
  * or opaque `Origin`) but does not refuse reverse-proxy forwarding headers:
  * those protect credentials and spend, which a keyless route has neither of,