build: the pin advances over the bounded calibration

libavr's calibrate() now bounds its measurement loop, starts the pulse
on an observed edge, and re-arms a rejected pulse on the remaining
budget instead of one-strike booting the application. The autobaud
images pay +16..20 B — every slot still fits, the worst now the 1284s'
502 of 512 — and the stock images are byte-identical, kept so by
fitting the loader's flag set per backend: -fno-ivopts stays on the
fixed-baud bodies it shrinks and comes off the autobaud body, where it
duplicated the calibration countdown into a 9-cycle loop against the
contracted seven.

One deployed constant moved and its gate caught it: the calibrate
wait's budget poll re-laid from ten cycles to nine (the exit branches
land where block layout puts them), so pureboot.window.autobaud
measured -10 % until AUTOBAUD_POLL_CYCLES and the README's derived
seconds were re-measured — the default autobaud window is 36 M cycles,
4.5 s at 8 MHz. Full gate green on all 37 chips; the README's autobaud
column carries each chip's rebuilt worst configuration, machine-checked
against the built trees.

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

test/pbwindow.py

@@ -27,8 +27,11 @@ sys.path.insert(0, str(pathlib.Path(__file__).resolve().parent))
 from pbsim import Device
 
 # The calibrate() budget loop's cycles per poll in the built image — what the
-# README's window arithmetic rests on, verified here.
-AUTOBAUD_POLL_CYCLES = 10
+# README's window arithmetic rests on, verified here. A measured fact, not a
+# design constant: the wait's exit branches land where the compiler's block
+# layout puts them, and the bounded-calibration rework moved the loop from
+# ten cycles to nine.
+AUTOBAUD_POLL_CYCLES = 9
 
 
 def load_tool(path):