pureboot 7: the same features in fewer words on every chip

Four cuts, none touching what the loader can do. The entry stub stops
re-doing the reset logic's own SP write where the datasheet guarantees
RAMEND (stack::hardware — the classic megas keep theirs). The autobaud
unit moves into GPIOR2:GPIOR1 wherever the chip has the pair: one-word
accesses, no RAM object, and the host's measured-clock peek follows it
by version and geometry. 'J' rides the unified decode, carrying a
selector it ignores so its address is the same two reads as every other
command — the tool sends the bare form to older residents. run_app stops
insisting on a body of its own. The fleet lands at 358–410 B stock and
438–474 B autobaud; the tightest image in the space — the 1284s'
autobaud on a USART's own pins with the OSCCAL trim — drops from 510 to
484 of its 512. Every chip's suite is green on the wire that changed,
and the README's table is machine-checked against the built images.

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

CMakeLists.txt

@@ -283,12 +283,14 @@ if(PROJECT_IS_TOP_LEVEL)
 	add_test(NAME pureboot_autobaud.size
 	         COMMAND ${CMAKE_COMMAND} -DSIZE_TOOL=${CMAKE_SIZE} -DELF=$<TARGET_FILE:pureboot_autobaud>
 	                 -DLIMIT=${PUREBOOT_LIMIT} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
-	# The measured unit is the loader's only RAM object and sits at the very
-	# start of SRAM — where the host reads the bit period from (--info's
-	# measured clock), so the address is wire contract, not layout accident.
+	# The measured unit's home is wire contract, not layout accident: the
+	# host reads the bit period from it (--info's measured clock). In the
+	# GPIOR home the image must carry no RAM copy at all; in the RAM home it
+	# is the loader's only RAM object, at the very start of SRAM.
 	add_test(NAME pureboot_autobaud.unit
 	         COMMAND ${CMAKE_COMMAND} -DOBJDUMP=${CMAKE_OBJDUMP} -DELF=$<TARGET_FILE:pureboot_autobaud>
-	                 -DRAM_START=${PUREBOOT_RAM_START} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_unit.cmake)
+	                 -DRAM_START=${PUREBOOT_RAM_START} -DGPIOR=${PUREBOOT_UNIT_GPIOR}
+	                 -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_unit.cmake)
 
 	# One point of the exhaustive matrix, named from its resolved parameters
 	# so the enumeration cannot collide with itself. `pins` is empty for the