feat(nestsClient): WebTransport abstraction + fake + Kwik stub

Phase 3b-1 of the Clubhouse/nests integration. Lands the
[WebTransportSession] abstraction the MoQ layer (Phase 3c) will code
against, so MoQ framing + tests can develop in parallel with the real
Kwik-based transport integration (deferred to Phase 3b-2).

commonMain:
- `WebTransportSession` — bidi/uni stream access, datagrams, close.
- `WebTransportBidiStream` / `WebTransportReadStream` /
  `WebTransportWriteStream` — minimal read/write surface.
- `WebTransportFactory.connect(authority, path, bearerToken)` for
  opening sessions.
- `WebTransportException(kind, ...)` with four canonical failure modes
  (HandshakeFailed, ConnectRejected, PeerClosed, NotImplemented) so UI
  code doesn't need to know about library-specific exceptions.
- `FakeWebTransport.pair()` — in-memory, fully-wired client/server
  pair for unit-testing MoQ framing without touching a real QUIC stack.

jvmAndroid:
- `KwikWebTransportFactory` stub that throws
  `WebTransportException(NotImplemented)` at `connect()`. Doc spells
  out the handshake sequence (QUIC dial → H3 SETTINGS →
  `:method=CONNECT :protocol=webtransport` Extended CONNECT → 2xx) so
  Phase 3b-2 can drop the real implementation in without touching
  callers.

Tests:
- `FakeWebTransportTest` — datagram round-trip both directions,
  bidi-stream write visible on peer side, close() flips isOpen.
- `WebTransportExceptionTest` — kind + message + cause preserved.
- `KwikWebTransportFactoryTest` — `connect()` currently fails with the
  NotImplemented sentinel, so Phase 3b-2 can replace this test when
  the real handshake lands.

https://claude.ai/code/session_013nVLALALKaHVgHm9u5Cg8D

nestsClient/src/jvmTest/kotlin/com/vitorpamplona/nestsclient/transport/KwikWebTransportFactoryTest.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.nestsclient.transport
+
+import kotlinx.coroutines.test.runTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+
+class KwikWebTransportFactoryTest {
+    @Test
+    fun connect_throws_NotImplemented_until_phase_3b_2() =
+        runTest {
+            val factory = KwikWebTransportFactory()
+            val ex =
+                assertFailsWith<WebTransportException> {
+                    factory.connect(
+                        authority = "nostrnests.com",
+                        path = "/moq",
+                        bearerToken = "ignored",
+                    )
+                }
+            assertEquals(WebTransportException.Kind.NotImplemented, ex.kind)
+        }
+}