Avoid wiping full error context on release

UPGRADING.md

@@ -132,6 +132,11 @@ One recursive lock covers both the pool and the registry, so error
 correctness there is worth more than throughput, but a program that raises
 errors in a hot loop will feel it.
 
+Releasing the last reference to a context remains serialized under that same
+pool lock, but it now resets only the handled/status/reported state, the string
+heads, and the stack-trace cursor. It no longer wipes the whole context buffer,
+so release is a fixed handful of stores rather than a tens-of-kilobytes write.
+
 The per-thread last-ditch context is a whole `akerr_ErrorContext` (tens of
 kilobytes) in thread-local storage, allocated per thread on first use of the
 library from that thread.