Draw curves in the terminal, and stop waiting five seconds to hear

`space` ticks runs on the serve dashboard's table and `enter` compares them:
one metric in braille, five distinct hues, and the table of what actually
differs under it. Both halves are routes that already existed — the browser's
own comparison endpoint for the curves, the runs export for the table, whose
input columns are filtered to the ones that vary.

The palette is hue rather than the web's lightness ramp on purpose: five steps
of one brand hue collapse to a single colour on a 16-colour tty.

The screen also subscribes to the engine's event bus over the same websocket a
browser uses, so a run that starts and finishes inside a tick is seen rather
than only recorded. `a` lists what a run left behind and fetches it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014SmyLMSqcJQ8tUL2qLj21s

backend/fluksio/sdk/stream.py

@@ -0,0 +1,85 @@
+"""The engine's event bus, read the way a browser reads it.
+
+Polling answers "what is true now", so a run that starts and finishes between
+two ticks was only ever a row in the history. This subscribes to the same
+websocket the dashboard in a browser does — nothing new is served for it.
+
+Blocking on purpose: `subscribe` is a thread's whole job, and the caller
+brings the thread. Reconnecting is this module's, since a dashboard outlives
+the engine it supervises.
+"""
+
+from __future__ import annotations
+
+import json
+import threading
+from collections.abc import Callable, Collection
+from typing import Any
+from urllib.parse import urlsplit, urlunsplit
+
+#: What a reconnect waits before trying again, growing to a minute — the shape
+#: the engine's own supervision backs off with.
+BACKOFF = (1.0, 5.0, 30.0, 60.0)
+
+#: How long a read waits before looking at `stop`. The socket is otherwise
+#: silent for as long as the house is, and a dashboard that is closing should
+#: not wait for the next event to notice.
+POLL_S = 1.0
+
+
+def socket_url(url: str, token: str) -> str:
+    """The websocket beside an engine's HTTP address.
+
+    The token goes in the query string because a handshake carries no headers
+    of its own — the same reason the browser sends it that way.
+    """
+    parts = urlsplit(url)
+    scheme = "wss" if parts.scheme == "https" else "ws"
+    return urlunsplit((scheme, parts.netloc, "/api/v1/flows/ws", f"token={token}", ""))
+
+
+def subscribe(
+    url: str,
+    token: str,
+    on_event: Callable[[dict[str, Any]], None],
+    stop: threading.Event,
+    kinds: Collection[str] | None = None,
+    on_error: Callable[[Exception], None] | None = None,
+) -> None:
+    """Hand every published event to `on_event` until `stop` is set.
+
+    `kinds` keeps only the event types named, which is worth doing here rather
+    than in the caller: `message_value` is most of what the bus carries and a
+    screen drawing runs wants none of it.
+
+    A failure is reported once per outage. A dashboard is the thing that stops
+    and starts the engine, so a reconnect loop announcing itself every second
+    would bury the log pane it shares.
+    """
+    from websockets.sync.client import connect
+
+    attempt = 0
+    while not stop.is_set():
+        try:
+            with connect(socket_url(url, token), open_timeout=POLL_S * 5) as socket:
+                attempt = 0
+                while not stop.is_set():
+                    try:
+                        frame = socket.recv(timeout=POLL_S)
+                    except TimeoutError:
+                        continue
+                    payload = json.loads(frame)
+                    events = (
+                        payload.get("events") or []
+                        if payload.get("type") == "batch"
+                        else [payload]
+                    )
+                    for event in events:
+                        if kinds is None or event.get("type") in kinds:
+                            on_event(event)
+                return
+        except Exception as exc:  # noqa: BLE001 — any transport failure retries
+            if attempt == 0 and on_error is not None:
+                on_error(exc)
+            stop.wait(BACKOFF[min(attempt, len(BACKOFF) - 1)])
+            attempt += 1