pureboot 7: the same features in fewer words on every chip

Four cuts, none touching what the loader can do. The entry stub stops
re-doing the reset logic's own SP write where the datasheet guarantees
RAMEND (stack::hardware — the classic megas keep theirs). The autobaud
unit moves into GPIOR2:GPIOR1 wherever the chip has the pair: one-word
accesses, no RAM object, and the host's measured-clock peek follows it
by version and geometry. 'J' rides the unified decode, carrying a
selector it ignores so its address is the same two reads as every other
command — the tool sends the bare form to older residents. run_app stops
insisting on a body of its own. The fleet lands at 358–410 B stock and
438–474 B autobaud; the tightest image in the space — the 1284s'
autobaud on a USART's own pins with the OSCCAL trim — drops from 510 to
484 of its 512. Every chip's suite is green on the wire that changed,
and the README's table is machine-checked against the built images.

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

test/pbautobaud.py

@@ -64,8 +64,9 @@ def main():
             for needed in ("version", "signature", "fuses", "verify:", "stays"):
                 if needed not in out:
                     fail(f"{label}: session output lacks {needed!r}\n{out}")
-            # The measured clock, decoded from the unit at ram_start. The
-            # runner's clock is exact, so the figure must land inside the
+            # The measured clock, decoded from the unit at whichever home this
+            # version keeps it in. The runner's clock is exact, so the figure
+            # must land inside the
             # encoding's own envelope: the loader floors the bit period to
             # 4-cycle spin granules after an 8-cycle discount, and the edge
             # poll can shave a few cycles more — one granule of slack below