pureboot 6: a build-time OSCCAL trim, applied ahead of every reset path

The RC-oscillator answer's device half (dev/tasks.md in libavr): OSCCAL joins
pureboot_add_loader() as one optional byte, written at the top of run() before
the WDRF bail so the watchdog hand-over inherits the corrected clock too.
Orthogonal to the backend — an autobaud build may carry it purely for the
application. No value, no code: the stock image differs from v5 in exactly
the version's two bytes (the stamp and the 'b' immediate).

Measured: +6 B where OSCCAL takes sts (328P, 404→410), +4 B in low I/O
(t85, 402→406); the tightest image in the space (1284 autobaud on USART
pins, 504) carries the sts form at 510 of 512. New gates: the OSCCAL size
points on every chip, the wire-observed trim byte on both addressing
classes (test/pbosccal.py, red-green), and the autobaud unit pinned to
ram_start (test/check_unit.cmake, red-green) — the address --info's
measured-clock read is about to rely on.

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

pureboot/CMakeLists.txt

@@ -136,6 +136,19 @@ elseif(LIBAVR_MCU STREQUAL "atmega644pa")
 	set(_pb_sim_mcu atmega644p)
 endif()
 
+# Where SRAM begins: the classic megas keep it right after the plain I/O
+# registers, the x8/x4 generations push it past their extended I/O file, and
+# the tinies match the classics. An autobaud loader's measured unit lives at
+# exactly this address (the host reads it there — pureboot.py), and the
+# unit-position test holds the layout to it.
+if(LIBAVR_MCU MATCHES "^atmega(8|16|32)a?$")
+	set(_pb_ram 0x60)
+elseif(LIBAVR_MCU MATCHES "^atmega")
+	set(_pb_ram 0x100)
+else()
+	set(_pb_ram 0x60)
+endif()
+
 # The function runs in its caller's scope, so everything it needs crosses
 # scopes as global properties.
 set_property(GLOBAL PROPERTY PUREBOOT_BASE_HEX ${_pb_base_hex})
@@ -155,6 +168,7 @@ set(PUREBOOT_SLOT ${_pb_slot} PARENT_SCOPE)
 set(PUREBOOT_LIMIT ${_pb_limit} PARENT_SCOPE)
 set(PUREBOOT_EEPROM ${_pb_eeprom} PARENT_SCOPE)
 set(PUREBOOT_DEFAULT_HZ ${_pb_hz} PARENT_SCOPE)
+set(PUREBOOT_RAM_START ${_pb_ram} PARENT_SCOPE)
 set(PUREBOOT_HAS_USART ${_pb_has_usart} PARENT_SCOPE)
 set(PUREBOOT_HAS_USART1 ${_pb_has_usart1} PARENT_SCOPE)
 set(PUREBOOT_SIM_MCU ${_pb_sim_mcu} PARENT_SCOPE)
@@ -212,7 +226,7 @@ endfunction()
 
 # pureboot_add_loader(<name> [CLOCK <hz>] [BAUD <bd>]
 #                     [SERIAL auto|hardware|software|autobaud] [USART <n>]
-#                     [RX <pin>] [TX <pin>] [TIMEOUT <s>])
+#                     [RX <pin>] [TX <pin>] [TIMEOUT <s>] [OSCCAL <byte>])
 #
 # The loader target plus its flashable images (<name>.hex for a programmer,
 # <name>.bin for --update-loader). The resolved deployment is stamped on the
@@ -225,8 +239,15 @@ endfunction()
 # and one binary per chip serves every F_CPU and every rate. The stamped
 # PUREBOOT_HZ/PUREBOOT_BAUD then record what a harness should *drive* it at,
 # not what it was built for.
+#
+# OSCCAL bakes a measured oscillator trim into the loader (README.md: the
+# RC-oscillator deployment answer): the byte is written at the top of run(),
+# so every reset path — the watchdog hand-over included — runs on the
+# corrected clock. Orthogonal to the backend: an autobaud build may carry it
+# purely for the application's benefit, its own link being clock-free. No
+# value, no code.
 function(pureboot_add_loader name)
-	cmake_parse_arguments(PB "" "CLOCK;BAUD;SERIAL;USART;RX;TX;TIMEOUT" "" ${ARGN})
+	cmake_parse_arguments(PB "" "CLOCK;BAUD;SERIAL;USART;RX;TX;TIMEOUT;OSCCAL" "" ${ARGN})
 	if(PB_UNPARSED_ARGUMENTS)
 		message(FATAL_ERROR "pureboot_add_loader(${name}): unknown arguments ${PB_UNPARSED_ARGUMENTS}")
 	endif()
@@ -325,6 +346,13 @@ function(pureboot_add_loader name)
 		set(_defines PUREBOOT_CLOCK_HZ=${PB_CLOCK} PUREBOOT_BAUD=${PB_BAUD} PUREBOOT_TIMEOUT=${PB_TIMEOUT}
 		    ${_serial_defines})
 	endif()
+	if(DEFINED PB_OSCCAL)
+		math(EXPR _osccal "${PB_OSCCAL}" OUTPUT_FORMAT DECIMAL)
+		if(_osccal LESS 0 OR _osccal GREATER 255)
+			message(FATAL_ERROR "pureboot_add_loader(${name}): OSCCAL ${PB_OSCCAL} is not one byte")
+		endif()
+		list(APPEND _defines PUREBOOT_OSCCAL=${_osccal})
+	endif()
 
 	add_executable(${name} ${CMAKE_CURRENT_FUNCTION_LIST_DIR}/pureboot.cpp)
 	target_link_libraries(${name} PRIVATE libavr)