Name every local error context errctx

Closes internal-consistency item 18. The convention was settled -- AGENTS.md
records errctx winning 92 sites to 45 -- but the other 45 were still there, and
four files favoured `e` throughout while six favoured errctx, sometimes within
one file.

Its own commit because it is a rename and nothing else. All 333 changed lines
are a single identifier substitution: applying \be\b -> errctx to each removed
line reproduces the added line exactly, and the counts match at 333 either way.

`e` keeps its meaning where the convention actually wants it -- an incoming
error context being inspected, as in akgl_get_json_with_default(e, ...) -- which
is why that function was left alone.

25/25 pass, reindent --check clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

src/heap.c

@@ -42,11 +42,11 @@ akerr_ErrorContext *akgl_heap_init(void)
 
 akerr_ErrorContext *akgl_heap_init_actor(void)
 {
-    PREPARE_ERROR(e);
+    PREPARE_ERROR(errctx);
     for ( int i = 0; i < AKGL_MAX_HEAP_ACTOR; i++) {
 	memset(&akgl_heap_actors[i], 0x00, sizeof(akgl_Actor));
     }
-    SUCCEED_RETURN(e);
+    SUCCEED_RETURN(errctx);
 }
 
 akerr_ErrorContext *akgl_heap_next_actor(akgl_Actor **dest)