pureboot: every deployment axis is a build parameter

Clock, baud, serial backend (hardware USART 0/1 or the software UART on
any pins) and the activation window all resolve through one CMake
function, pureboot_add_loader() in pureboot/CMakeLists.txt — the unit a
downstream project consumes. The default baud is the fastest standard
rate within 2.5 % (the same best-divisor search libavr's solver runs),
gated on software builds by the polled receiver's 100-cycles-a-bit
floor; every explicit pick is re-checked by the compile's static asserts.

The size matrix builds each axis that can move the image — backend x
clock ladder x USART instance, per chip — against the slot budget, and
two nondefault deployments run the whole protocol suite live: the 328P
on its shipped 1 MHz fuses over software serial on TX=PB1/RX=PB5
(pureboot.custom), and the 644A over USART1 (pureboot.usart1). The sim
runner takes -l to bridge any link, paces a fully quiet bridge toward
real time (a free-running 8 M-cycle window loses the reset-race knock),
and the fixture application speaks the deployment it is built for.

The loader itself shed bytes on the way: the return-address high byte
spelled through byteswap (the double swap folds to the one-byte pick),
the info-block address composed instead of bit_cast, and libavr's new
polled-UART helpers replacing the port's uart::detail reaches. Every
combination fits: 458-506 B across the megas' whole matrix, 470-484 B
on the tinies, 556-562 B in the 1284s' 1 KiB slot.

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

test/pbsim.py

@@ -8,8 +8,11 @@ import subprocess
 
 
 class Device:
-    def __init__(self, binary, elf, mcu, hz, base_hex, page, baud, dump, reset_hex=None, resume=None):
-        cmd = [binary, elf, mcu, hz, base_hex, str(page), str(baud), dump]
+    def __init__(self, binary, elf, mcu, hz, base_hex, page, baud, dump, reset_hex=None, resume=None, link=None):
+        cmd = [binary]
+        if link:
+            cmd += ["-l", link]
+        cmd += [elf, mcu, hz, base_hex, str(page), str(baud), dump]
         if reset_hex is not None or resume is not None:
             # Chips without a hardware boot section — the tinies and the
             # m48s — reset to address 0 like silicon; the boot-sectioned