Use malloc zone for Darwin realloc hook

[fallintoplace]

Motivation

The Darwin allocation-counting hook for malloc_zone_realloc accepted a malloc zone, but delegated to plain realloc. That ignored the zone argument and could reallocate through the wrong allocator for zone-specific allocation paths.

Modification

Call malloc_zone_realloc(zone, ptr, size) from replacement_malloc_zone_realloc.

This also adds a small Darwin C executable test target with a custom malloc zone. The test verifies that replacement_malloc_zone_realloc dispatches through the requested zone's realloc callback.

Result

Darwin zone-specific realloc calls now preserve the requested malloc zone while keeping the existing allocation/free counter behavior.

Validation

• swift run --package-path IntegrationTests/allocation-counter-tests-framework/template/HookedFunctionsDoHook HookedFunctionsDarwinTests
• swift build --package-path IntegrationTests/allocation-counter-tests-framework/template/HookedFunctionsDoHook
• git diff --check