pureboot 6: a build-time OSCCAL trim, applied ahead of every reset path

The RC-oscillator answer's device half (dev/tasks.md in libavr): OSCCAL joins
pureboot_add_loader() as one optional byte, written at the top of run() before
the WDRF bail so the watchdog hand-over inherits the corrected clock too.
Orthogonal to the backend — an autobaud build may carry it purely for the
application. No value, no code: the stock image differs from v5 in exactly
the version's two bytes (the stamp and the 'b' immediate).

Measured: +6 B where OSCCAL takes sts (328P, 404→410), +4 B in low I/O
(t85, 402→406); the tightest image in the space (1284 autobaud on USART
pins, 504) carries the sts form at 510 of 512. New gates: the OSCCAL size
points on every chip, the wire-observed trim byte on both addressing
classes (test/pbosccal.py, red-green), and the autobaud unit pinned to
ram_start (test/check_unit.cmake, red-green) — the address --info's
measured-clock read is about to rely on.

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

pureboot/pureboot.py

@@ -24,13 +24,15 @@ else:
     import termios
 
 PROMPT = b"+"
-VERSION = 5  # this tool's own version — free to drift from a loader's
-# The loader versions this tool speaks. A pureboot version implies its wire
+VERSION = 6  # this tool's own version — free to drift from a loader's
+# The loader versions this tool can drive. A pureboot version implies its wire
 # protocol, which carries no number of its own, so this window is where that
-# map lives: every version so far speaks the same protocol, and one that
-# changes it becomes the new floor here.
+# map lives: the tool keeps a decoder for every generation in it (1–4 speak
+# the per-memory commands, 5 the unified pair; 6 marks the OSCCAL-carrying
+# builds and changes nothing on the wire), and a version it has no decoder
+# for moves the floor.
 OLDEST_LOADER = 1
-NEWEST_LOADER = 5
+NEWEST_LOADER = 6
 SLOT = 512  # the loader slot, on every chip
 RETRIES = 3  # rewrites of a page that reads back wrong, before the run stops