Fix refcount leak and stack-trace buffer overflow

Two memory-safety bugs in the macro core:

1. Refcount leak. ENSURE_ERROR_READY incremented refcount on every FAIL/SUCCEED
   rather than only when it acquired a fresh context from the pool. A function
   that FAILed a context more than once and then propagated arrived at its
   caller with refcount 2; the caller released once, leaking the slot. After
   AKERR_MAX_ARRAY_ERROR leaks the pool is exhausted and the library exit(1)s.
   Move the increment inside the acquisition branch.

2. Stack-trace overflow. Each appended frame passed the full buffer length to
   snprintf instead of the space remaining, and advanced the cursor by
   snprintf's would-be return value, so a trace that filled the buffer wrote
   past the end of stacktracebuf and ran the cursor out of bounds. Add
   AKERR_STACKTRACE_APPEND, which bounds the write to the remaining space and
   clamps the cursor advance.

Regression tests err_refcount_double_fail and err_stacktrace_bounds fail against
the old code (verified) and pass now. Full suite: 21/21, no warnings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

CMakeLists.txt

@@ -67,6 +67,8 @@ set(AKERR_TESTS
     err_release_clears
     err_pool_exhaust
     err_maxval
+    err_refcount_double_fail
+    err_stacktrace_bounds
 )
 
 set(AKERR_WILL_FAIL_TESTS