Take libakerror 2.0.1 and drop the workaround it makes obsolete

Every libakgl test suite could report success while failing. libakerror's
default unhandled-error handler ended in exit(errctx->status), an exit
status is one byte wide, and libakgl's band starts at 256 -- so
AKGL_ERR_SDL, the most common failure a library built on SDL can have,
exited 0 and CTest recorded a pass. tests/character.c aborted at its
second of four tests on a bad renderer and was green for months.

0.5.0 worked around that here with TEST_TRAP_UNHANDLED_ERRORS() in
tests/testutil.h, and TODO.md ended the entry saying any consumer's
suites have the same problem and it was worth raising upstream. It was.
2.0.1 fixes it at the source: akerr_exit() owns the mapping and the
default handler calls it, so 0 exits 0, 1 through 255 exit themselves,
and anything else exits AKERR_EXIT_STATUS_UNREPRESENTABLE (125). The
trap and its 21 call sites are gone.

Verified by putting the original failure back rather than by reading the
release notes: a FAIL_BREAK(AKGL_ERR_SDL) in tests/character.c's main
exits 125 and CTest reports a failure. A standalone consumer raising the
same status unhandled exits 125 where it exited 0 before.

tests/actor.c installs its own handler and called exit(errctx->status)
from it, which is the same defect one layer up. It calls akerr_exit()
now.

2.0.0 also makes the error pool and the status registry thread safe,
which libakgl needs more than it knew: audio_stream_callback raises
error contexts on SDL's audio thread. With an unlocked pool that
callback and the main thread could scan AKERR_ARRAY_ERROR at the same
time and be handed the same slot. The comment there says so.

This is a hard dependency floor, not a preference. 2.0.0 moved
__akerr_last_ignored to thread-local storage and made akerr_next_error()
return a context that already holds its reference, and both expand at
libakgl's call sites -- and at a consumer's, because akerror.h is part
of libakgl's public interface. Mixing headers and libraries across that
line double-counts every reference and never returns a pool slot. The
soname moved to libakerror.so.2; include/akgl/error.h now also feature-
tests AKERR_EXIT_STATUS_UNREPRESENTABLE, which is the narrowest probe
for 2.0.1 since libakerror publishes no version macro.

0.7.0 for that reason: libakgl's own ABI is unchanged, but the one it
re-exports through its headers is not.

TODO.md records the pkg-config gap this makes sharper -- akgl.pc names
no dependencies at all, so nothing tells a pkg-config consumer which
libakerror it needs.

Clean build, 26/26 ctest, memcheck clean, warning-clean at -Wall
-Werror. libakgl.so.0.7 links libakerror.so.2.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B8T5FAYXE8HEJqFLCYwNNc

include/akgl/error.h

@@ -24,7 +24,20 @@
  * See deps/libakerror/UPGRADING.md.
  */
 #ifndef AKERR_FIRST_CONSUMER_STATUS
-#error "libakgl requires libakerror >= 1.0.0: the akerror.h on the include path predates the status registry. Rebuild and reinstall libakerror."
+#error "libakgl requires libakerror >= 2.0.1: the akerror.h on the include path predates the status registry. Rebuild and reinstall libakerror."
+#endif
+// 2.0.0 is an ABI break -- akerr_next_error() returns a context that already
+// holds its reference, and __akerr_last_ignored is thread-local -- and both of
+// those expand at *this* library's call sites through IGNORE and the FAIL
+// macros. A libakgl built against a 1.x header and linked against 2.x
+// double-counts every reference and never returns a slot to the pool. The
+// soname moved to libakerror.so.2 so the two cannot be mixed by accident, but
+// the header can still be stale in an install tree, so check it here too.
+//
+// AKERR_EXIT_STATUS_UNREPRESENTABLE arrived in 2.0.1 and is the narrowest thing
+// to probe for: libakerror publishes no version macro.
+#ifndef AKERR_EXIT_STATUS_UNREPRESENTABLE
+#error "libakgl requires libakerror >= 2.0.1: the akerror.h on the include path predates akerr_exit(). Rebuild and reinstall libakerror."
 #endif
 
 // Silences -Wformat-truncation on a string concatenation that genuinely may not