pureboot moves to its own repo

pureboot is at git.blackmark.me/avr/pureboot now, with its own history: the
files it kept in pureboot/ sit at the top level there, its CMakeLists is the
merged whole of that unit and the build around it, and the v1..v8 tags moved
with it - each one checks out and compiles to exactly the .text it compiled to
here. The loader that repo builds is byte-identical to the one this commit
removes, verified before the removal rather than after.

Nothing is rewritten on this side. The history and the tags are untouched, so
every commit before this one still has pureboot in it and still builds it;
this is one commit that stops carrying it forward.

What goes with it: the four pureboot files, the thirteen pb*.py protocol
drivers and their four host-side unit tests, pbapp and the pureboot simavr
runner, check_pi.py, pbhw.py, pbrig.py, sizes.py, and the Studio project.
check_unit.cmake goes too - it had no caller left once the unit tests moved.

What is left is the four TinySafeBoot tiers, and the build shrinks to fit
them: 757 lines of CMakeLists to 137, and the preset matrix from 37 chips to
one, because the tiers reimplement an ATmega328P-only protocol and every other
chip in that list was there for pureboot. check.sh loses its size-table pass -
the table it checked was pureboot's README - and the Studio solution loses the
project that is now in the other repo.

Gate green: nine tests, four tiers at their section sizes and each one's
protocol suite against the simulator.

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

tools/check.sh

@@ -1,15 +1,13 @@
 #!/bin/bash
-# The port's gate: every chip's generated workflow - build, size matrix, and
-# the simulator-driven protocol suites. --full adds the reflect-spot builds
-# (libavr's rule: reflect compiles are bounded to its spot set, never the
-# full matrix) and swaps the compact size matrix for the exhaustive
-# clock x baud x backend cross product. libavr resolves from the `libavr/`
-# submodule; LIBAVR_ROOT overrides it for a working tree.
+# The port's gate: the generated workflow - build, size tests, and the
+# simulator-driven protocol suite. --full adds the reflect build, which
+# compiles the same TUs through libavr's other producer. libavr resolves from
+# the `libavr/` submodule; LIBAVR_ROOT overrides it for a working tree.
 set -e
 cd "$(dirname "$0")/.."
 
 full=0
-[[ "$1" == "--full" ]] && { full=1; shift; export PUREBOOT_FULL_MATRIX=1; }
+[[ "$1" == "--full" ]] && { full=1; shift; }
 
 # The chip lists come from the presets rather than being spelled a second time
 # here: a chip added to make_presets.py and missed in a copy of its list would
@@ -59,10 +57,4 @@ if ((${#red[@]})); then
 	exit 1
 fi
 
-# Every tree is freshly built now - the one moment the README's size table
-# can be held to what the images measure (a per-preset ctest sees only its
-# own chip; the table needs all of them, and ungated it drifts: a
-# common-code shave moves every row at once with nothing over budget).
-python3 tools/sizes.py check-readme
-
 echo "check: every chip green"