pureboot: the 1284P rides a 1 KiB slot — its own boot-sector minimum

The far machinery (ELPM reads, RAMPZ page commands, wire-word math)
costs ~46 B over the m328P's 504, and the tsb-calibrated C++-to-asm
gap says no implementation of this feature set reaches 512 on this
chip — a boundary its hardware does not have anyway: the 1284P's
smallest boot sector is 1 KiB. The slot therefore becomes
per-geometry (512 B, or 1 KiB past 64 KiB), which the host derives
from the word-addressing flag; slot arithmetic unifies (the index is
the wire high byte with its low bit dropped in either unit), the
update preflight demands a two-slot boot section in the chip's own
terms, and pbapp's hand-back jumps to the real slot base. libavr's
far primitives split their RAMPZ/Z asm operands (a page never
crosses 64 KiB, so callers keep a byte and a 16-bit cursor — the
32-bit address folds away; flash_load_far's byte form becomes the
out-RAMPZ+elpm pair avr-libc's pgm_read_byte_far rebuilds per call),
and the host splits reads at 64 KiB boundaries. All ten chips pass
the full suite — the 1284P at 558 B including protocol, relocation,
and the power-fail self-update — with pureboot byte-identical across
generated and reflect modes everywhere, and the original three
chips' images unchanged to the byte (488/502/504).

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

test/pbreloc.py

@@ -24,7 +24,7 @@ def fail(message):
 def main():
     device_bin, elf, mcu, hz, base_hex, page, baud, tool, workdir = sys.argv[1:]
     base, page, baud = int(base_hex, 0), int(page), int(baud)
-    stage = base - 512
+    stage = None  # derived from the device's own info (slot-sized) below
     sys.path.insert(0, os.path.dirname(os.path.abspath(tool)))
     sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
     import pbsim
@@ -46,6 +46,7 @@ def main():
         resident_info = info.raw
 
         # Install the staging copy exactly as the update flow would.
+        stage = info.stage
         staged = pb.staging_content(image, info)
         pb.write_differing(loader, stage, staged)
 
@@ -78,7 +79,7 @@ def main():
 
         # Restore the resident image through the staged copy, then 'J' back
         # into it and prove it lives.
-        resident = image + b"\xff" * (512 - len(image))
+        resident = image + b"\xff" * (info.slot - len(image))
         pb.write_differing(loader, base, resident)
         back_info = loader.enter_copy(base, 25)
         if back_info.raw != resident_info: