review: the port's findings — the version map speaks v7, costs told true

The in-file version window now carries the v7 line its own comment
claimed to hold; the GPIOR note counts words, not instructions; the
USART-release cost and citation match the silicon (two bytes on the
classics, §20.6.3); and the 512-byte claim reads as the slot bound it
is. The libavr pin advances over the review pass — images byte-identical.

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

pureboot/pureboot.py

@@ -42,6 +42,10 @@ RETRIES = 3  # rewrites of a page that reads back wrong, before the run stops
 # 'g' writes, each taking a selector byte, a 16-bit address and a count, over
 # the spaces below. The loader carries one transfer loop instead of four bodies
 # — which is what buys the data space and the host-issued SPM operations.
+# 6 marks the builds that may carry a baked OSCCAL trim, nothing on the wire;
+# 7 gives 'J' a selector byte (older loaders take the bare address — jump()
+# sends each form to the version that speaks it) and re-homes the autobaud
+# unit into the GPIOR pair where the chip has one.
 UNIFIED_LOADER = 5
 SP_FLASH, SP_EEPROM, SP_RAM, SP_FUSE, SP_SPM = 0, 1, 2, 3, 4