Take libakerror 2.0.1, and guard the exit status it fixes

2.0.0 makes the error pool and the status registry thread safe, and it is an ABI
break carrying the soname to libakerror.so.2. The break is a quiet one:
__akerr_last_ignored became thread-local and akerr_next_error() now returns a
context that already holds a reference, so objects compiled against a 1.x header
and linked against 2.x count every reference twice and never give a slot back.
Nothing about that fails to link, which is exactly what a guard is for --
include/akbasic/error.h feature-tests AKERR_THREAD_SAFE instead of
AKERR_FIRST_CONSUMER_STATUS, which 2.0.0 also still defines and which therefore
no longer distinguishes anything.

2.0.1 is the release this band needed most. The default unhandled-error handler
ended in exit(errctx->status), and a process exit status is one byte:
AKBASIC_ERR_BASE is 512, and 512 truncates to 0, so an unhandled
AKBASIC_ERR_SYNTAX reported success to anything watching $?. Every other code in
the band came out as some unrelated error's number. akerr_exit() substitutes 125
for anything a byte cannot carry, and a probe raising AKBASIC_ERR_DEVICE through
FINISH_NORETURN now exits 125 rather than 7.

It was latent here rather than live -- src/main.c handles the context and returns
EXIT_FAILURE, and every test with a top-level ATTEMPT carries a HANDLE_DEFAULT --
but "no caller relies on it today" is not a property a header can keep true.
tests/version_check.c asserts the mapping and fails if AKBASIC_ERR_BASE ever
stops truncating to zero, because that is the day this stops being about our base.

Chapter 10 gains a threading section: libakerror is safe from any thread now, and
this interpreter is not and has no lock anywhere in it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -138,8 +138,9 @@ API documentation builds with `doxygen Doxyfile`, into `build/docs/html`.
 Everything is a submodule; `git submodule update --init --recursive` gets all of it. There is
 nothing to install first.
 
-* [libakerror](https://source.starfort.tech/andrew/libakerror) 1.0.0 — TRY/CATCH-style error
-  contexts. Every function that can fail returns one.
+* [libakerror](https://source.starfort.tech/andrew/libakerror) 2.0.1 — TRY/CATCH-style error
+  contexts. Every function that can fail returns one. 2.0.0 made it thread safe and broke the
+  ABI; anything built against a 1.x header must be rebuilt rather than relinked.
 * [libakstdlib](https://source.starfort.tech/andrew/libakstdlib) 0.2.0 — libc wrappers that
   report through `libakerror`. String-to-number conversion goes straight to it, which is why
   `VAL("garbage")` is an error rather than a silent `0`.