tool: survive our own leftovers — drain the fresh port, shorten the identity read

--stay leaves the loader's final prompt in the USB pipeline; a fresh
invocation on a board that resets when its port opens then flushes too
early, trusts the stale prompt, and spends the new activation window on
a 2-second identity read against a device that never heard its knock —
collecting the application's banner as an unknown signature. Three
host-side moves, no device bytes: the line is drained until quiet
(bounded, 250 ms) before the port's first knock — once per port, since a
mid-session re-knock faces no foreign bytes and its own window is
already burning; the identity read_exact drops 2.0 to 0.5 s, dozens of
times the worst real answer, so any false prompt match leaves room for
the retry that already works; and the tool version drifts to 8. The
StaleDTRPort fixture models the whole moment — stale prompt in transit,
reset holding the device off the line, a finite window, the banner —
red against the old tool in exactly the field shape (unknown signature
from banner bytes), green now; LoaderPort answers its prompt to the
knock rather than to a read count, which the drain exposed as a
call-order coupling.

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

pureboot/pureboot.py

@@ -26,7 +26,7 @@ else:
     import termios
 
 PROMPT = b"+"
-VERSION = 7  # this tool's own version — free to drift from a loader's
+VERSION = 8  # this tool's own version — free to drift from a loader's
 # The loader versions this tool can drive. A pureboot version implies its wire
 # protocol, which carries no number of its own, so this window is where that
 # map lives: the tool keeps a decoder for every generation in it (1–4 speak
@@ -546,6 +546,11 @@ class Loader:
         # Set once a session is established over an autobaud link, so a
         # re-entry after 'J' repeats the handshake that worked.
         self.autobaud = False
+        # The pre-knock drain runs once per port: the bytes it exists for are
+        # leftovers from before this process opened the port. Re-knocks later
+        # in the same session must not pay it — a fresh activation window is
+        # already burning while they wait.
+        self._line_drained = False
         # The link this session is speaking. It moves when the host follows a
         # staging copy built for another one (enter_copy).
         self.baud = getattr(port, "baud", None)
@@ -555,10 +560,16 @@ class Loader:
         """The 'b' reply, in either of the two layouts a loader may send.
         pureboot 5 answers with its version and the signature; older loaders
         answer with a 12-byte block. The version byte cannot be mistaken for
-        the older block's 'P', so four bytes are enough to tell them apart."""
-        head = self.port.read_exact(4, 2.0)
+        the older block's 'P', so four bytes are enough to tell them apart.
+
+        The timeout is short on purpose: a real answer follows the prompt
+        within a frame time or two, so half a second is dozens of times the
+        worst case — while a *false* prompt match (a stale byte, reset
+        garbage) makes this read collect noise, and every second spent on it
+        comes out of the activation window the retry needs."""
+        head = self.port.read_exact(4, 0.5)
         if head[0:2] == b"PB":
-            return Info(head + self.port.read_exact(8, 2.0))
+            return Info(head + self.port.read_exact(8, 0.5))
         return Info.from_identity(head)
 
     def _handshake(self, wait, knock, what):
@@ -569,8 +580,23 @@ class Loader:
         into a fresh window, where a command without its knock is discarded.
         Each attempt is therefore the whole handshake. This also converges into
         an already-live session: the knock bytes are ignored there and the
-        drain absorbs whatever they produced."""
+        drain absorbs whatever they produced.
+
+        Before the port's first knock ever, the line is drained until quiet: a
+        prompt from a previous session (`--stay`) can still be in the USB
+        pipeline when the port opens, where a flush cannot clear what has not
+        arrived yet — and on a board that resets when its port opens, trusting
+        that stale byte would spend the fresh activation window reading noise
+        from a device that never heard the knock. Once only, and bounded:
+        later re-knocks in this session face no foreign leftovers, and their
+        own window is already burning."""
         deadline = time.monotonic() + wait
+        if not self._line_drained:
+            self._line_drained = True
+            drain = time.monotonic() + 0.25
+            while self.port.read_available(0.05):
+                if time.monotonic() > drain:
+                    break
         knocks = 0
         refusal = None
         while True: