pureboot.py: bound the activation drain against a flooding target

The post-prompt settle loop in _handshake had no deadline, so a target that
never falls quiet — a board stuck in a reset loop, whose UART-reset garbage
carries a stray prompt byte — spun the tool forever. Bound it by the handshake
deadline; a real loader still settles on its first quiet read. Regression:
test/test_handshake.py (flood terminates, valid loader still connects).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pureboot/pureboot.py

@@ -484,8 +484,13 @@ class Loader:
             self.port.write(knock)
             knocks += 1
             if PROMPT in self.port.read_available(0.4):
+                # Settle: absorb a real loader's trailing bytes before asking
+                # for the identity. Bounded by the deadline so a target that
+                # never falls quiet — a board stuck in a reset loop, whose
+                # garbage carries a stray prompt — cannot spin here forever.
                 while self.port.read_available(0.3):
-                    pass
+                    if time.monotonic() > deadline:
+                        break
                 self.port.write(b"b")
                 try:
                     # A version the tool cannot speak is the loader's own