feat(odoh): reject relay+target sharing an eTLD+1

Plain host-string equality caught the copy-paste-same-URL footgun but
let `r.cloudflare.com` + `odoh.cloudflare.com` through — two subdomains
of the same operator collapse ODoH to ordinary DoH. Add a second layer:
compare registrable domains via the PSL (`psl` crate) after the exact-
host check. Fails open on IP literals and unparseable hosts; the exact-
host check still runs in those cases.

Cargo.lock

@@ -1562,6 +1562,7 @@ dependencies = [
  "hyper-util",
  "log",
  "odoh-rs",
+ "psl",
  "qrcode",
  "rand_core 0.9.5",
  "rcgen",
@@ -1802,6 +1803,21 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "psl"
+version = "2.1.203"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "76c0777260d32b76a8c3c197646707085d37e79d63b5872a29192c8d4f60f50b"
+dependencies = [
+ "psl-types",
+]
+
+[[package]]
+name = "psl-types"
+version = "2.0.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "33cb294fe86a74cbcf50d4445b37da762029549ebeea341421c7c70370f86cac"
+
 [[package]]
 name = "qrcode"
 version = "0.14.1"