test: start the e2e suite for socket protocol 2

A pytest suite run by hand against a device (test/e2e/README.md): hello, the schema and its hash, the sync with next and resync, replies to bad frames, feed subscriptions, the status event, every client syncing at once, a sixth client refused, a page load, and the socket under the device's security mode. It never transmits, changes settings or reboots. --record saves hello, the sync and the schema as fixtures.

PROTOCOL.md now describes the protocol as built.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/e2e/test_survival.py

@@ -0,0 +1,68 @@
+"""The device must survive the loads that ran its heap out
+(docs/adr/0005-one-websocket-protocol.md): every client reconnecting at once,
+and a browser loading the page."""
+import threading
+import urllib.request
+
+import pytest
+
+from proto_client import ProtoClient, heap, run_sync
+
+# What the heap may drop to under load.  Below it the next allocation in the
+# web server is close to failing, which aborts.
+HEAP_FLOOR = 20 * 1024
+MAX_CLIENTS = 5
+
+
+@pytest.fixture
+def watch_heap(host):
+    """Fails the test when the device rebooted or its heap fell under the floor."""
+    before = heap(host)
+    yield before
+    after = heap(host)
+    assert after["uptime"] > before["uptime"], "the device rebooted"
+    assert after["min"] >= HEAP_FLOOR or after["min"] == before["min"], (
+        "the heap fell to %u bytes (floor %u)" % (after["min"], HEAP_FLOOR))
+
+
+def test_every_client_reconnecting_at_once(connect, schema, watch_heap):
+    clients = [connect() for _ in range(MAX_CLIENTS)]
+    errors = []
+
+    def sync(c):
+        try:
+            run_sync(c, timeout=60)
+        except Exception as e:  # reported below
+            errors.append(e)
+    threads = [threading.Thread(target=sync, args=(c,)) for c in clients]
+    [t.start() for t in threads]
+    [t.join() for t in threads]
+    assert not errors, errors
+
+
+def test_one_client_too_many_is_closed(connect, schema):
+    clients = [connect(schema=schema.hash) for _ in range(MAX_CLIENTS)]
+    for c in clients:
+        run_sync(c)
+    extra = connect(schema=schema.hash)
+    assert extra.closed.wait(5), "a sixth client stayed connected"
+
+
+def test_page_load(host, token, connect, watch_heap):
+    """The static files in parallel, as a browser fetches them, then the socket."""
+    files = ["/", "/main.css", "/widgets.css", "/icons.css", "/index.js", "/icon.png"]
+    errors = []
+
+    def get(path):
+        try:
+            req = urllib.request.Request(f"http://{host}{path}", headers={"Accept-Encoding": "gzip"})
+            with urllib.request.urlopen(req, timeout=60) as r:
+                r.read()
+        except Exception as e:  # reported below
+            errors.append((path, e))
+    threads = [threading.Thread(target=get, args=(p,)) for p in files]
+    [t.start() for t in threads]
+    c = connect()
+    run_sync(c, timeout=60)
+    [t.join() for t in threads]
+    assert not errors, errors