pureboot: artifact roles, wrong-chip refusal, and misplaced-loader re-homing

The README's deployment section now says what each build artifact is for:
the .hex is the programmer artifact (self-addressed into the top slot),
the .bin the self-update image — bare slot bytes a programmer would put
at address 0, where a boot-sectioned mega cannot even heal itself (SPM
only runs from the boot section) but a patched-vector chip runs the
position-independent copy and re-homes a build through the ordinary
--update-loader flow: the staging install and the word-0 redirect both
execute outside page 0's slot, so the running-slot guard never blocks it.
pbrehome.py is the acceptance test (misplaced at 0, guard intact,
re-home, app flash over the stale copy, banner); the staging slot is the
one position that cannot re-home itself, documented. The preflight's
wrong-chip refusal and loader_image's handling of padded images (peeled
to the slot content by the embedded base) are documented and the padded
case pinned in the planner.

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

CMakeLists.txt

@@ -305,6 +305,20 @@ if(PROJECT_IS_TOP_LEVEL)
 		set_tests_properties(pureboot.reloc PROPERTIES TIMEOUT 180
 		                     ENVIRONMENT "PB_OBJCOPY=${CMAKE_OBJCOPY}")
 
+		# Re-homing: a loader mistakenly programmed at address 0 (a raw .bin
+		# handed to a programmer) must heal into the canonical slot through
+		# the ordinary --update-loader flow. Patched-vector behavior, so one
+		# representative chip carries it.
+		if(LIBAVR_MCU STREQUAL "attiny85")
+			add_test(NAME pureboot.rehome
+			         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbrehome.py
+			                 ${PB_DEVICE} $<TARGET_FILE:pureboot> $<TARGET_FILE:pureboot9>.bin ${_pb_sim_mcu}
+			                 ${_pb_hz} ${_pb_base_hex} ${_pb_page} ${_pb_baud} $<TARGET_FILE:pbapp>.bin
+			                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+			                 ${CMAKE_BINARY_DIR}/pbrehome-work)
+			set_tests_properties(pureboot.rehome PROPERTIES TIMEOUT 180)
+		endif()
+
 		# The self-update end-to-end: the re-timed build (same source, only
 		# PUREBOOT_TIMEOUT differs — a byte-different image) replaces the
 		# resident through --update-loader, with every power-fail phase