Point the site and the harnesses at the four repositories

The relay and the Kotlin engine are published on their own now, so the
self-hosting guide told people to clone a directory that no longer exists
and the source page said everything was in one repository.

Both pages list all four now, with what each is for. The self-hosting guide
still only needs the first one -- the rest are there so that self-hostable
is something a reader can check rather than something they are told.

The Android vector loader hardcoded ../../engine/test/vectors, which is only
right inside this repository. It resolves from a candidate list now, honours
FLAMENET_VECTORS, and still FAILS rather than skips when it finds nothing:
the entire value of a third engine is that it is checked against something
other than a reading of our own code. Nine tests pass against the JDK in
Homebrew, which is not on the default java_home path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

engine/src/test/kotlin/io/flamenet/e2e/PublishedVectorsTest.kt

@@ -24,10 +24,30 @@ import kotlin.test.assertTrue
 class PublishedVectorsTest {
 
     private val vectors: JSONObject by lazy {
-        // From android/engine/ up to the repository root.
-        val file = File("../../engine/test/vectors/protocol-v1.json")
-        assertTrue(file.exists(), "published vectors not found at ${file.absolutePath}")
-        JSONObject(file.readText())
+        // Where the vectors are depends on which checkout this is: android/ as a
+        // subtree of the app repo, or the extracted Android repo with
+        // flamenet-e2e cloned beside it. Resolved rather than assumed, because a
+        // hardcoded depth is how the harness stops testing anything.
+        //
+        // This FAILS rather than skips when it finds nothing, deliberately: the
+        // whole reason a third engine is worth writing is that it is checked
+        // against the published vectors, and one that quietly stops checking is
+        // worth less than no third engine at all.
+        val candidates = listOfNotNull(
+            System.getenv("FLAMENET_VECTORS"),
+            "../../engine/test/vectors/protocol-v1.json",       // app repo subtree
+            "../../../flamenet-e2e/test/vectors/protocol-v1.json", // beside the engine repo
+            "../../flamenet-e2e/test/vectors/protocol-v1.json",
+        ).map { File(it) }
+        val file = candidates.firstOrNull { it.exists() }
+        assertTrue(
+            file != null,
+            "published vectors not found. Looked in: " +
+                candidates.joinToString(", ") { it.absolutePath } +
+                ". Clone https://gitlab.com/paulhitt/flamenet-e2e beside this repo, " +
+                "or set FLAMENET_VECTORS to protocol-v1.json.",
+        )
+        JSONObject(file!!.readText())
     }
 
     private fun JSONArray.bytesAt(i: Int) = Bytes.unb64(getString(i))