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>

UPGRADING.md

@@ -1,3 +1,38 @@
+# Bug fix: the AKERR_USE_STDLIB=OFF build, and a pool-exhaustion exit code (2.0.2)
+
+`-DAKERR_USE_STDLIB=OFF` did not compile at all ([issue #12](https://source.starfort.tech/andrew/libakerror/issues/12)):
+`bool`, `PATH_MAX` and `NULL` were used unconditionally in the public header
+but only included under the stdlib branch, and the CMake option itself was
+pasted straight into a preprocessor definition, so a boolean spelling like
+`ON` silently evaluated to 0 in `#if AKERR_USE_STDLIB == 1`. Both are fixed:
+`<stdbool.h>` and `<stddef.h>` are now included unconditionally (they are
+freestanding-safe), and the CMake option is normalized to a plain `1`/`0`
+before being stamped into the header.
+
+`AKERR_USE_STDLIB=OFF` now has a mandatory, explicit contract:
+`AKERR_RUNTIME_HEADER` must name a header providing `exit`, `memset`,
+`snprintf`, `strcmp`, `strlen` and `strncpy` — the header `#error`s at compile
+time naming those six symbols if it is unset. `AKERR_THREADS` must resolve to
+`none`; the configure now fails outright (not a warning) if it would resolve
+to `pthread`, since the pthread backend calls into libc. See
+[docs/building.md](docs/building.md#dependencies).
+
+`PATH_MAX`, used to size the `fname`/`function` fields of `akerr_ErrorContext`,
+is retired in favor of a new `AKERR_MAX_ERROR_FNAME_LENGTH` build option. Its
+default (4096) matches `PATH_MAX` on Linux/glibc, so `sizeof(akerr_ErrorContext)`
+and the soname are unchanged unless you deliberately override it — **no ABI
+break**.
+
+**Behavior change, not an ABI break:** `ENSURE_ERROR_READY`'s pool-exhaustion
+path — reached when every slot in `AKERR_ARRAY_ERROR` is checked out and
+something still tries to raise — used to call `exit(1)` directly. It now calls
+`akerr_exit(AKERR_EXIT_STATUS_UNREPRESENTABLE)`, the same terminal path every
+other exit out of the library's status space goes through, so **the exit code
+for that case changes from 1 to 125**. If you were checking `$?` for exactly
+`1` to detect pool exhaustion specifically, check for 125 instead (and note
+125 is shared with every other status an exit code cannot carry — see the note
+on `AKERR_EXIT_STATUS_UNREPRESENTABLE` in the header).
+
 # Bug fix: unhandled-error exit status (2.0.1)
 
 An unhandled error could kill the process and still report success.