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>

include/akerror.tmpl.h

@@ -1,12 +1,42 @@
 #ifndef _AKERR_H_
 #define _AKERR_H_
 
-#if (defined(AKERR_USE_STDLIB) && AKERR_USE_STDLIB == 1) || (!defined(AKERR_USE_STDLIB))
-#include <stdlib.h>
+/*
+ * A consumer compiling this header directly (not through the CMake package,
+ * which always stamps a numeric AKERR_USE_STDLIB=0/1 -- see CMakeLists.txt)
+ * gets the hosted default.
+ */
+#ifndef AKERR_USE_STDLIB
+#define AKERR_USE_STDLIB 1
+#endif
+
+/*
+ * <stdbool.h> and <stddef.h> are freestanding-safe (C99/C11 4p6): they define
+ * only bool/true/false and NULL/size_t, nothing that requires an operating
+ * system underneath. Include them unconditionally so both configurations get
+ * those types. Everything that actually talks to a hosted environment --
+ * stdlib.h, string.h, stdio.h -- stays behind AKERR_USE_STDLIB.
+ */
 #include <stdbool.h>
+#include <stddef.h>
+
+#if AKERR_USE_STDLIB
+#include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
-#include <limits.h>
+#else
+/*
+ * Freestanding runtime contract. AKERR_USE_STDLIB=OFF still needs exit,
+ * memset, snprintf, strcmp, strlen and strncpy (see the FAIL/ENSURE_ERROR_READY
+ * macros and src/error.c below) -- this library does not implement its own
+ * copies of them. Define AKERR_RUNTIME_HEADER to a header that provides all
+ * six before including this one.
+ */
+#ifdef AKERR_RUNTIME_HEADER
+#include AKERR_RUNTIME_HEADER
+#else
+#error "AKERR_USE_STDLIB is OFF: define AKERR_RUNTIME_HEADER to a header providing exit, memset, snprintf, strcmp, strlen and strncpy"
+#endif
 #endif
 
 /*
@@ -52,7 +82,15 @@
 #define AKERR_MAX_ERROR_CONTEXT_STRING_LENGTH              12384
 
 #define AKERR_MAX_ERROR_NAME_LENGTH                        64
-#define AKERR_MAX_ERROR_FNAME_LENGTH                       PATH_MAX
+/*
+ * scripts/generrno.sh stamps this in from the AKERR_MAX_ERROR_FNAME_LENGTH
+ * CMake cache variable, the same way it stamps AKERR_THREAD_SAFE and
+ * AKERR_LAST_ERRNO_VALUE. It used to be PATH_MAX, which is not available in a
+ * freestanding build; the default here (4096) matches PATH_MAX on Linux/glibc
+ * so sizeof(akerr_ErrorContext) and the soname are unchanged unless you
+ * deliberately override it.
+ */
+#define AKERR_MAX_ERROR_FNAME_LENGTH                       4096
 #define AKERR_MAX_ERROR_FUNCTION_LENGTH                    128
 #define AKERR_MAX_ERROR_STACKTRACE_BUF_LENGTH              (AKERR_MAX_ERROR_CONTEXT_STRING_LENGTH + AKERR_MAX_ERROR_NAME_LENGTH + AKERR_MAX_ERROR_FNAME_LENGTH + AKERR_MAX_ERROR_FUNCTION_LENGTH + 16)
 
@@ -296,7 +334,7 @@ akerr_ErrorContext AKERR_NOIGNORE *__akerr_copy_string(char *destination, int ca
 	__err_context = akerr_next_error();				\
 	if ( __err_context == NULL ) {					\
 	    akerr_log_method("%s:%s:%d: Unable to pull an error context from the array!", __FILE__, (char *)__func__, __LINE__); \
-	    exit(1);							\
+	    akerr_exit(AKERR_EXIT_STATUS_UNREPRESENTABLE);		\
 	}								\
     }