Add a background frame pump so QMP load is constant

One thread grabs the LCD at a fixed rate into a shared buffer; HTTP clients serve
from that buffer. Before, every /stream iteration issued its own QMP reads, so
load scaled with client count and reconnects, and a slow reader could stall the
grab loop.

Encodes only when the framebuffer bytes change, and exposes generation so a client
can tell a new frame from the same one without comparing bytes.

rebind() is here for the power work in a later task: the emulator comes and goes
under the server, and rebind(None) blanks the screen instead of leaving a stale
frame that looks live. The run loop re-checks the grabber identity under the lock
after a read, so a rebind landing mid-read cannot be undone by the frame it was
already fetching.

A client that raises is swallowed on purpose -- a dead emulator must not kill the
pump, since power may come back.

Verified against a real emulator: 5000 latest() calls in 1 ms with no QMP traffic,
generation flat at 1 over a second of static screen, guest still running, and
rebind(None) going dark. 9 unit tests.

tools/test_uvk5_stream.py

@@ -0,0 +1,134 @@
+#!/usr/bin/env python3
+"""Unit tests for the background frame pump. No emulator needed."""
+import tempfile
+import threading
+import time
+import unittest
+
+from uvk5_stream import FramePump
+
+
+class CountingClient:
+    """Counts memsave calls, so QMP load can be asserted."""
+
+    def __init__(self):
+        self.reads = 0
+        self.lock = threading.Lock()
+
+    def command(self, name, **args):
+        if name == "pmemsave":
+            raise AssertionError("pmemsave reads physical addresses; use memsave")
+        if name != "memsave":
+            raise AssertionError(f"unexpected {name}")
+        with self.lock:
+            self.reads += 1
+        with open(args["filename"], "wb") as fh:
+            fh.write(bytes(args["size"]))
+        return {}
+
+
+def wait_for_frame(pump, timeout=3.0):
+    deadline = time.monotonic() + timeout
+    while time.monotonic() < deadline:
+        if pump.latest() is not None:
+            return True
+        time.sleep(0.01)
+    return False
+
+
+class TestFramePump(unittest.TestCase):
+    def _pump(self, client=None, fps=50):
+        pump = FramePump(client if client is not None else CountingClient(),
+                         0x1000, 0x2000, fps=fps, spool_dir=tempfile.mkdtemp())
+        self.addCleanup(pump.stop)
+        return pump
+
+    def test_latest_returns_a_png_once_started(self):
+        pump = self._pump()
+        pump.start()
+        self.assertTrue(wait_for_frame(pump), "no frame produced")
+        self.assertTrue(pump.latest().startswith(b"\x89PNG\r\n\x1a\n"))
+
+    def test_latest_is_none_before_start(self):
+        pump = self._pump()
+        self.assertIsNone(pump.latest())
+
+    def test_read_rate_is_independent_of_reader_count(self):
+        """QMP load must not scale with clients: that is the whole point."""
+        client = CountingClient()
+        pump = self._pump(client, fps=20)
+        pump.start()
+        self.assertTrue(wait_for_frame(pump))
+
+        time.sleep(0.5)
+        with client.lock:
+            first = client.reads
+        # Hammer latest() the way many readers would. It must cause no reads.
+        for _ in range(2000):
+            pump.latest()
+        with client.lock:
+            after_hammer = client.reads
+        self.assertLess(after_hammer - first, 30,
+                        "serving frames must not trigger fresh QMP reads")
+
+    def test_stop_halts_reading(self):
+        client = CountingClient()
+        pump = self._pump(client)
+        pump.start()
+        self.assertTrue(wait_for_frame(pump))
+        pump.stop()
+        with client.lock:
+            settled = client.reads
+        time.sleep(0.3)
+        with client.lock:
+            self.assertEqual(client.reads, settled)
+
+    def test_generation_does_not_advance_on_an_unchanged_frame(self):
+        """Encoding only on change is what keeps idle CPU near zero."""
+        pump = self._pump()
+        pump.start()
+        self.assertTrue(wait_for_frame(pump))
+        time.sleep(0.2)
+        gen = pump.generation()
+        time.sleep(0.3)
+        # The stub always returns the same zero frame, so nothing changed.
+        self.assertEqual(pump.generation(), gen)
+
+    def test_start_is_idempotent(self):
+        pump = self._pump()
+        pump.start()
+        pump.start()
+        self.assertTrue(wait_for_frame(pump))
+
+    def test_survives_a_client_that_raises(self):
+        """A dead emulator must not kill the pump; power may come back."""
+        class Broken:
+            def command(self, name, **args):
+                raise RuntimeError("emulator gone")
+
+        pump = self._pump(Broken())
+        pump.start()
+        time.sleep(0.2)
+        self.assertIsNone(pump.latest())      # nothing to show, but still alive
+        pump.stop()
+
+    def test_rebind_to_none_blanks_the_screen(self):
+        """Power off must go dark, not keep showing a stale frame."""
+        pump = self._pump()
+        pump.start()
+        self.assertTrue(wait_for_frame(pump))
+        pump.rebind(None)
+        self.assertIsNone(pump.latest())
+
+    def test_rebind_to_a_client_resumes(self):
+        pump = self._pump()
+        pump.start()
+        self.assertTrue(wait_for_frame(pump))
+        pump.rebind(None)
+        self.assertIsNone(pump.latest())
+        pump.rebind(CountingClient())
+        self.assertTrue(wait_for_frame(pump), "did not resume after rebind")
+
+
+if __name__ == "__main__":
+    unittest.main()