test: the device runners speak the C++ the rest of the repo does

pureboot_device and the tsb device, C until now, rewritten in C++23 with
every modeled behavior intact — the PGERS Z-mask and m48-discard ioctl
wraps, the GPIO bridge's timing and pacing, the tiny NVM's write-once
buffer, pin ownership, and the PB_PTY/TSB_PTY lines the harnesses parse.
The one linkage fact worth a comment: simavr's parts headers (uart_pty.h)
carry no C++ guards where its core headers do, so those includes sit in an
extern "C" block. Warning-clean at -Wall -Wextra on the build line; the
full protocol suites on all four sim-driven chips prove the conversion.

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

pureboot/README.md

@@ -551,7 +551,7 @@ Per chip preset, `ctest` runs:
   (`tools/make_presets.py --check`), so a hand edit or a generator change
   cannot drift the pair apart;
 - `pureboot.protocol` — end to end against a simavr device
-  (`test/pureboot_device.c`: a hardware USART as a pty, or a cycle-timed
+  (`test/pureboot_device.cpp`: a hardware USART as a pty, or a cycle-timed
   GPIO⇄pty bridge for a software-UART build, plus the SPM/NVM module simavr's
   tiny cores lack) driven by the real host tool through knock-from-reset,
   program + verify of both memories, session reconnect, an external reset