pureboot 3: a 512-byte slot on every chip, the 1284s included

The word-addressed 1284s were the one family deploying in a 1 KiB slot,
because the far-flash machinery (ELPM reads, RAMPZ page commands, a
word-addressed wire) did not fit 512 B. It does now: 478 B stock, 494 B in
the heaviest configuration the build can produce. They take the 644s'
geometry, where the smallest boot section holds the resident slot and its
staging slot together. The loader's version goes to 3; the host tool did
not change, so its own version stays 2 and only the window it speaks
widens.

Most of the saving is one restructure. The info block and a flash read are
the same act, so giving all four streamed commands one address-and-count
path leaves exactly one call site for the flash streamer: it inlines into
the never-returning command loop and its 24-bit cursor stops being saved
and restored around every transmit. Around it, the ack byte moved out of
line, the wire's byte pair is bit_cast into the word it already is, the
fuse loop ends on its count, the info block's in-slot offset is taken as
the one-byte relocation it is, and -fno-expensive-optimizations gives way
to -fno-move-loop-invariants -fno-tree-ter. Every chip shrank 14-18 B.

The size matrix grew the axes it was missing: the USART1 instance across
the whole clock ladder, and the shape a slow baud gives a software UART —
past 255 delay iterations libavr takes the 16-bit delay loop, which the
ladder default never selects and which was 4 B over the 1284's slot the
first time it was built.

The protocol fixture stopped deriving the loader entry from the flash
size; on the 1284s it had been jumping a slot low and reaching the loader
only because erased flash walked it up.

Docs and comments were consolidated across the port in the same pass: the
README carries a per-chip size table instead of prose, and prose that
restated the code is gone — 190 lines, no behaviour with it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/check_pi.py

@@ -1,17 +1,11 @@
 #!/usr/bin/env python3
-"""Position-independence lint for the pureboot image.
+"""Position-independence lint: the two link-time facts that let the identical
+image run from any slot, asserted from the built ELF.
 
-The self-staging design lets the identical binary run from any 512-byte
-slot, which holds only if nothing in the image addresses itself absolutely.
-Two link-time facts guarantee it, both asserted here from the built ELF:
-
-  1. No absolute jmp/call opcodes — all control flow is PC-relative
-     (rjmp/rcall/ijmp/icall). -mrelax normally guarantees this; a code
-     change that grows a branch out of relaxation range would break it
-     silently.
-  2. The info block sits within the image's first 256 bytes: the 'b'
-     command rebuilds its address as (running slot high byte : low byte of
-     the link address), which needs the offset to fit that low byte.
+  1. No absolute jmp/call — -mrelax normally guarantees it, but a branch that
+     grows out of relaxation range would break it silently.
+  2. The info block within the image's first 256 bytes: 'b' rebuilds its
+     address as (running slot high byte : link address low byte).
 
 Usage: check_pi.py <objdump> <nm> <elf> <text_start_hex>
 """