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>

ide/README.md

@@ -2,7 +2,7 @@
 
 `master` carries `bootloader.atsln`, so this branch does too: `ide/bootloader.atsln`
 builds the loaders from the same sources Ninja does, to a **byte-identical
-`.text`** — 400 B for the 328P pureboot loader, 510 B for the `tsb_asm` tier in
+`.text`** — 390 B for the 328P pureboot loader, 510 B for the `tsb_asm` tier in
 its 512-byte section. CMake remains the build system; the solution is here so the
 port opens in Studio as its predecessor did.