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>

test/check_unit.cmake

@@ -0,0 +1,23 @@
+# Asserts the autobaud loader's measured unit is the first RAM object: the
+# host tool reads the bit period from ram_start (--info's measured clock), so
+# the unit's address is wire contract. Run as
+#   cmake -DOBJDUMP=... -DELF=... -DRAM_START=<data address> -P check_unit.cmake
+
+execute_process(COMMAND ${OBJDUMP} -t ${ELF} OUTPUT_VARIABLE _syms RESULT_VARIABLE _res)
+if(NOT _res EQUAL 0)
+	message(FATAL_ERROR "${OBJDUMP} -t ${ELF} failed")
+endif()
+
+# The symbol line: "00800100 l     O .noinit  00000002 <mangled>unit_E".
+string(REGEX MATCH "\n0*([0-9a-f]+)[^\n]+[ \t][^ \t\n]*unit_[^ \t\n]*\n" _line "${_syms}")
+if(NOT _line)
+	message(FATAL_ERROR "no unit_ symbol in ${ELF} — is this the autobaud loader?")
+endif()
+
+# AVR data-space symbols carry the 0x800000 VMA offset.
+math(EXPR _want "0x800000 + ${RAM_START}" OUTPUT_FORMAT HEXADECIMAL)
+math(EXPR _have "0x${CMAKE_MATCH_1}" OUTPUT_FORMAT HEXADECIMAL)
+if(NOT _have STREQUAL _want)
+	message(FATAL_ERROR "unit_ sits at ${_have}, ram_start is ${_want} — the host peeks ram_start")
+endif()
+message(STATUS "unit_ at ${_have} == ram_start")

test/pbosccal.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+"""The build-time OSCCAL trim, observed through the wire: a loader built with
+the OSCCAL axis holds the trim register at the built byte from its first
+prompt on — the write sits at the top of run(), ahead of the WDRF bail, so
+every path out of reset runs on the corrected clock. simavr's clock does not
+follow OSCCAL, which is what makes the value assertable at all: the register
+is plain state there, and the peek must return exactly what the build
+declared rather than whatever the oscillator needed.
+
+Usage: pbosccal.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
+                   <baud> <osccal_addr> <osccal_value> <tool_py> <workdir>
+                   [link]
+"""
+
+import os
+import sys
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def main():
+    args = sys.argv[1:]
+    link = args.pop() if len(args) == 12 else None
+    (device_bin, elf, mcu, hz, base_hex, page, baud, addr, value, tool, workdir) = args
+    addr, value, baud = int(addr, 0), int(value, 0), int(baud)
+    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+    import pbsim
+
+    os.makedirs(workdir, exist_ok=True)
+    dump = os.path.join(workdir, "flash_dump.bin")
+    device = pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump, link=link)
+    try:
+        out = pbsim.run_tool(tool, device.pty, baud, "--peek", f"{addr:#x}:1")
+        want = f"{addr:#06x}  {value:02x}"
+        if want not in out:
+            fail(f"OSCCAL at {addr:#x} did not read back {value:#04x}:\n{out}")
+    finally:
+        device.stop()
+    print("OK")
+
+
+if __name__ == "__main__":
+    main()