Stop returning past CLEANUP, and validate the arguments every sibling validates

Closes internal-consistency items 16 and 17.

Ten *_RETURN macros sat inside ATTEMPT blocks, which return past CLEANUP and
skip every release in it. The one that mattered was the success path of
akgl_get_json_tilemap_property: it leaked two of the string pool's 256 entries
on every lookup that *found* what it was asked for, and a map load does that
several times per layer. tests/tilemap.c now runs each of its three paths --
found, absent, wrong type -- twice the pool size and asserts the pool is where
it started. Against the old code that test does not merely fail, it segfaults,
which is Defects item 30 seen from the outside: pool exhaustion arriving as a
NULL strncpy rather than as AKGL_ERR_HEAP.

Two of the conversions needed more than swapping the macro. In
akgl_get_json_tilemap_property a plain break would have fallen through to the
"property not found" FAIL_RETURN after FINISH, reporting a miss for something
found, so the success path sets a flag. In akgl_collide_rectangles the eight
early exits are followed by `*collide = false;`, which would have overwritten
the hit that broke out; each corner test writes the flag itself, so that line
is gone rather than moved. The same function also released its scratch string
once per loop iteration while continuing to use it, so the slot was free while
still live -- one claim now covers the whole scan.

akgl_controller_default is the other behavioural one: its SUCCEED_RETURN was
the last statement in the ATTEMPT block, so the path that falls out of FINISH
reached the closing brace of a non-void function.

scripts/check_error_protocol.py keeps both rules enforced -- a *_RETURN inside
ATTEMPT, and a return out of a HANDLE block -- as the error_protocol test.
Neither produces a compiler diagnostic and neither fails a test run until the
pool it drains is empty, which is why both have already shipped once.

For item 17: the eight typed JSON accessors that validated their container and
then wrote through dest unconditionally now check key and dest as the two
string accessors always did; the null physics backend checks its actors like
the arcade one; and akgl_render_2d_frame_start, _frame_end and _shutdown check
self, which the first two read straight through. tests/renderer.c calls all
three with NULL, which segfaulted before.

25/25 pass, memcheck clean, reindent --check clean.

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

tests/testutil.h

@@ -20,6 +20,28 @@
 #include <SDL3/SDL.h>
 #include <akerror.h>
 #include <akgl/error.h>
+#include <akgl/heap.h>
+
+/**
+ * @brief How many pool string slots are currently claimed.
+ *
+ * The string pool is 256 entries and every loader borrows scratch strings from
+ * it, so "does this function give back what it took" is a question a test can
+ * answer directly by counting either side of a call. A leak that would take
+ * fifty map loads to show up in production shows up here on the first one.
+ */
+static inline int test_string_pool_used(void)
+{
+    int used = 0;
+    int i = 0;
+
+    for ( i = 0; i < AKGL_MAX_HEAP_STRING; i++ ) {
+	if ( akgl_heap_strings[i].refcount != 0 ) {
+	    used += 1;
+	}
+    }
+    return used;
+}
 
 /**
  * @brief Exit with a status the shell can actually see.