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>

.gitea/workflows/ci.yaml

@@ -37,6 +37,34 @@ jobs:
           fail_on_failure: 'true'
       - run: echo "🍏 This job's status is ${{ job.status }}."
 
+  # Builds and tests the AKERR_USE_STDLIB=OFF configuration end to end (issue
+  # #12), and separately proves that the generated header compiles under a
+  # genuinely freestanding toolchain (-nostdinc -ffreestanding, no libc at
+  # all) via tests/freestanding_fixture.c, which is never linked or run.
+  cmake_build_freestanding:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository code
+        uses: actions/checkout@v4
+      - name: dependencies
+        run: |
+          sudo apt-get update -y
+          sudo apt-get install -y cmake gcc
+      - name: configure, build and test (AKERR_USE_STDLIB=OFF)
+        run: |
+          cmake -S . -B build-off -DAKERR_USE_STDLIB=OFF -DAKERR_THREADS=none
+          cmake --build build-off
+          ctest --test-dir build-off --output-on-failure
+      - name: freestanding consumer fixture (compile-only, no libc)
+        run: |
+          gcc -c -std=c11 \
+              -nostdinc -ffreestanding \
+              -isystem "$(gcc -print-file-name=include)" \
+              -I build-off/generated/include \
+              -I tests \
+              tests/freestanding_fixture.c -o /tmp/freestanding_fixture.o
+      - run: echo "🍏 This job's status is ${{ job.status }}."
+
   coverage:
     runs-on: ubuntu-latest
     steps: