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>

test/pbrehome.py

@@ -0,0 +1,78 @@
+#!/usr/bin/env python3
+"""Re-homing acceptance test: a pureboot image mistakenly programmed at
+address 0 of a patched-vector chip (a raw .bin handed to a programmer, which
+defaults to offset 0) must still be a working loader — position-independent,
+guarding its accidental slot — and the ordinary --update-loader flow must
+re-home a build into the canonical top slot: the staging install and the
+word-0 redirect both run from copies whose slots are not page 0's, so the
+running-slot guard never blocks the flow. Flashing an application through
+the healed resident then overwrites the stale copy, vector surgery included,
+and the banner proves the launch.
+
+Usage: pbrehome.py <device_bin> <pureboot_elf> <update_bin> <mcu> <hz>
+                   <base_hex> <page> <baud> <app_bin> <tool_py> <workdir>
+"""
+
+import os
+import sys
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def main():
+    (device_bin, elf, update_bin, mcu, hz, base_hex, page, baud, app_bin, tool, workdir) = sys.argv[1:]
+    base, baud = int(base_hex, 0), int(baud)
+    sys.path.insert(0, os.path.dirname(os.path.abspath(tool)))
+    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+    import pbsim
+    import pureboot as pb
+
+    os.makedirs(workdir, exist_ok=True)
+    dump = os.path.join(workdir, "dump.bin")
+    state = os.path.join(workdir, "rehome.pbstate")
+    if os.path.exists(state):
+        os.unlink(state)
+
+    # The runner's base argument places the firmware: 0 is the misplaced
+    # flash, and the patched-vector reset at word 0 lands in it directly.
+    device = pbsim.Device(device_bin, elf, mcu, hz, "0x0", int(page), baud, dump, reset_hex="0")
+    try:
+        port = pb.Port(device.pty, baud)
+        loader = pb.Loader(port)
+        info = loader.connect(25)
+        if info.base != base:
+            fail(f"the misplaced copy reports base {info.base:#06x} — the info block must stay canonical")
+
+        # The accidental slot still guards itself; the canonical base slot
+        # must be writable from it (that is what re-homing rides on).
+        before = loader.read_flash(0, info.page)
+        loader.write_page(0, bytes(info.page))
+        if loader.read_flash(0, info.page) != before:
+            fail("the misplaced copy's guard let its own slot change")
+
+        # The ordinary update flow re-homes the build into the top slot.
+        pb.op_update_loader(loader, 25, update_bin, state, None)
+        update = open(update_bin, "rb").read()
+        if loader.read_flash(base, len(update)) != update:
+            fail("the canonical slot does not hold the update image")
+
+        # An application flashed through the healed resident overwrites the
+        # stale copy (surgery included) and launches.
+        pages = pb.plan_flash(open(app_bin, "rb").read(), loader.info)
+        for address in pb.covered(pages, loader.info, skip_blank=False):
+            loader.write_page(address, pages[address])
+        pb.verify_pages(loader, pages)
+        loader.run_application()
+        if port.read_exact(3, 5.0) != b"APP":
+            fail("application does not banner after the re-home")
+        port.close()
+    finally:
+        device.stop()
+    print("pbrehome: a misplaced loader re-homes through the ordinary update flow")
+
+
+if __name__ == "__main__":
+    main()