fix(server): admit reverse-proxied same-origin terrain requests

Review follow-up on #975. The shared same-site gate refuses any forwarding header, which is right for routes that spend a key, but terrain is keyless and needed for rendering: behind the reverse proxy the README documents, every /api/terrain/heights chunk got 403 and fell back to the geoid. Terrain now uses admitSameSiteKeyless, which keeps the Sec-Fetch-Site and exact-Origin checks (comparing Origin against a single X-Forwarded-Proto scheme) but does not refuse forwarding headers. The shared admitSameSite gate is unchanged for every other route. The changelog entry moves to changelog.d/975-terrain-cross-site-gate.md.

Co-authored-by: daikaginza <[email protected]>

src/proxyErrorResponses.test.mjs

@@ -29,7 +29,7 @@ function fixture(name, overrides = {}, preview = false) {
     makeRateLimiter, clientKey,
     // terrain.js gates cross-site requests first; fixture requests carry no
     // browser headers, so admit them like a loopback tool.
-    admitSameSite: () => false,
+    admitSameSiteKeyless: () => false,
     path, process: { cwd: () => '/fixture', env: {} },
     fsp: {
       readFile: async () => { throw new Error('cache absent'); },