build: the libavr pin advances over the drain and delay contracts

The hand-over drains move to the explicit drain_unbounded() — both link
adapters drain only after their own write, so the frame is in flight by
construction and the bounded default's countdown would be dead bytes;
the images stay byte-identical. window_polls() states its arithmetic
through dev::cycles_for with the whole window converted before the
per-poll division — one truncation instead of one per second, same
instructions, only the countdown's immediate moves. Every size in the
matrix is unchanged; the full gate is green on all 37 chips.

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

pureboot/pureboot.cpp

@@ -10,6 +10,8 @@
 // is what makes a copy one slot below able to rewrite the resident one, and
 // every change here has to keep it (test/check_pi.py).
 
+#include <chrono>
+
 #include <libavr/libavr.hpp>
 
 using namespace avr::literals;
@@ -209,7 +211,10 @@ struct hardware_link {
 
 	static void drain()
 	{
-		uart::drain();
+		// A drain here always follows this link's own write — the frame is
+		// in flight by construction, so the completion the wait needs is
+		// guaranteed and the bounded default's countdown would be dead bytes.
+		uart::drain_unbounded();
 	}
 };
 
@@ -277,7 +282,10 @@ struct autobaud_link {
 
 	static void drain()
 	{
-		uart::drain();
+		// A drain here always follows this link's own write — the frame is
+		// in flight by construction, so the completion the wait needs is
+		// guaranteed and the bounded default's countdown would be dead bytes.
+		uart::drain_unbounded();
 	}
 };
 
@@ -340,7 +348,10 @@ void await_host()
 // cost stays wide, so the choice cannot oscillate on the boundary.
 consteval std::uint32_t polls_at(std::uint32_t per_poll)
 {
-	return timeout_seconds * (dev::clock.hz / per_poll);
+	// Whole-window cycles first, then the per-poll division: one truncation
+	// instead of one per second. Same instructions either way — only the
+	// countdown's immediate moves.
+	return static_cast<std::uint32_t>(dev::cycles_for<std::chrono::seconds{timeout_seconds}>() / per_poll);
 }
 
 consteval bool narrow_window()