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>

pureboot/CMakeLists.txt

@@ -376,9 +376,18 @@ function(pureboot_add_loader name)
 	# load-immediate it saves. The set is fitted to the loader's body and has to
 	# be re-measured when that body changes: -fno-move-loop-invariants belonged
 	# here while the command loop carried four transfer bodies and costs bytes
-	# now that it carries one.
-	target_compile_options(${name} PRIVATE
-	    -fno-ivopts -fira-algorithm=priority -fno-tree-ter -fno-split-wide-types)
+	# now that it carries one, and -fno-ivopts is fitted per backend — an
+	# autobaud body needs ivopts to keep the calibration countdown a single
+	# induction variable (without it the counter is duplicated and the
+	# measurement loop runs 9 cycles instead of its contracted 7), while the
+	# fixed-baud bodies still measure smaller with it off.
+	if(PB_SERIAL STREQUAL "autobaud")
+		target_compile_options(${name} PRIVATE
+		    -fira-algorithm=priority -fno-tree-ter -fno-split-wide-types)
+	else()
+		target_compile_options(${name} PRIVATE
+		    -fno-ivopts -fira-algorithm=priority -fno-tree-ter -fno-split-wide-types)
+	endif()
 	target_link_options(${name} PRIVATE -nostartfiles -Wl,--section-start=.text=${_base_hex}
 	                    -Wl,--defsym=pureboot_app=${_app} ${_wrap})
 	add_custom_command(TARGET ${name} POST_BUILD COMMAND ${CMAKE_SIZE} $<TARGET_FILE:${name}>)