Define the small-object allocator state as an ordinary global

`ScrSaState scr_sa;` was a tentative definition, which the runtime build
emits as a COMMON symbol. Library localization (abi.localize_runtime)
deliberately keeps COMMON symbols global (the ASan image-registration guard
relies on that), so every runtime-localized library archive exported
scr_sa next to its declared API, failing tests/harness/library-multi.test.ts
M1 ("external definitions equal the declared set exactly").

Initialize it explicitly so it is a regular zero-initialized definition that
localization demotes like every other runtime global. No other global added
by the stack is a tentative definition. The library-multi suite passes
(it fails M1 without this change).

packages/runtime/src/scr_alloc.c

@@ -14,7 +14,11 @@
 #if SCR_SMALL_ALLOC
 #include <sys/mman.h>
 
-ScrSaState scr_sa;
+/* Explicitly initialized so it is an ordinary (localizable) definition: a
+ * tentative definition becomes a COMMON symbol, which library localization
+ * deliberately keeps global (abi.localize_runtime), leaking it from every
+ * localized library archive. */
+ScrSaState scr_sa = {0};
 static bool scr_sa_refused;
 
 /* Reserve address space for every class slice at once; smaller slices are