Fix the AKERR_USE_STDLIB=OFF build (issue #12)

The freestanding build (-DAKERR_USE_STDLIB=OFF) did not compile at all:
bool, PATH_MAX and NULL were used unconditionally in the public header
but only included under the stdlib branch, and the CMake option was
pasted straight into a preprocessor definition, so a non-numeric cache
spelling (-DAKERR_USE_STDLIB=ON) silently evaluated to 0.

- Normalize AKERR_USE_STDLIB to a plain 1/0 in CMake before stamping it,
  and use a consistent '#if AKERR_USE_STDLIB' everywhere it is tested.
- Include <stdbool.h>/<stddef.h> unconditionally (freestanding-safe);
  keep <stdlib.h>/<string.h>/<stdio.h> behind AKERR_USE_STDLIB; move
  <limits.h> out of the public header into src/error.c, its only user.
- Define the freestanding runtime contract: AKERR_RUNTIME_HEADER must
  name a header providing exit, memset, snprintf, strcmp, strlen and
  strncpy when AKERR_USE_STDLIB is OFF, or the header #errors naming
  them. Add cmake/akerr_default_runtime.h, a libc-backed convenience
  default so this repo's own OFF build and tests work out of the box.
- Route ENSURE_ERROR_READY's pool-exhaustion path through akerr_exit()
  instead of a direct exit(1). Deliberate behavior change: that exit
  code moves from 1 to AKERR_EXIT_STATUS_UNREPRESENTABLE (125), the same
  sentinel every other unrepresentable status already uses. Documented
  in docs/building.md and UPGRADING.md. Not an ABI break.
- Retire PATH_MAX: AKERR_MAX_ERROR_FNAME_LENGTH is now stamped by
  scripts/generrno.sh from a new AKERR_MAX_ERROR_FNAME_LENGTH cache
  variable, defaulting to 4096 (PATH_MAX on Linux/glibc) so
  sizeof(akerr_ErrorContext) and the soname are unchanged. Rewrite the
  now-stale PATH_MAX justification in src/lock.h's feature-test-macro
  comment.
- Fail the configure with a FATAL_ERROR, not a warning, when
  AKERR_USE_STDLIB=OFF and AKERR_THREADS would resolve to pthread,
  naming -DAKERR_THREADS=none as the fix.
- Keep the generated errno table (errno.c) out of the OFF build; skip
  the 'errno --list' shellout in scripts/generrno.sh under OFF and
  stamp AKERR_LAST_ERRNO_VALUE from a new fallback cache variable
  (default 133, Linux's EHWPOISON) instead.
- Update docs/building.md: drop the known-defect paragraph, fix
  sprintf -> snprintf, add size_t, drop PATH_MAX, and document the new
  options and the exit-code change.
- Guard tests/err_errno.c's registered-name assertion behind
  AKERR_USE_STDLIB: akerr_init_errno() is not called when it is OFF.
- Add a CI job that configures/builds/tests AKERR_USE_STDLIB=OFF with
  AKERR_THREADS=none, plus a compile-only -nostdinc -ffreestanding
  check of tests/freestanding_fixture.c against the generated header.

Bump the project version to 2.0.2 (no ABI break: soname and struct
layout are unchanged).

Verified locally: OFF+none configures and builds clean with all tests
passing; ON and the plain default build both build and pass their full
test suites (37/37); OFF with the default/auto thread backend fails
configure with a message naming -DAKERR_THREADS=none; sizeof(akerr_ErrorContext)
is unchanged (37296 bytes, fname/function still 4096 each) versus the
pre-change tree.

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

tests/freestanding_fixture.c

@@ -0,0 +1,14 @@
+/*
+ * Compile-only proof that a genuinely freestanding consumer (-nostdinc
+ * -ffreestanding, no libc) can include the generated header under
+ * AKERR_USE_STDLIB=OFF. Never linked or run -- see .gitea/workflows/ci.yaml.
+ */
+#define AKERR_USE_STDLIB 0
+#define AKERR_RUNTIME_HEADER "freestanding_fixture_runtime.h"
+#include "akerror.h"
+
+akerr_ErrorContext *akerr_freestanding_fixture_example(void)
+{
+    PREPARE_ERROR(e);
+    FAIL_RETURN(e, AKERR_VALUE, "freestanding fixture example error");
+}