fix(0110): for-over-array by-value fetch reads one element, not a full copy

lowerFor's by-value element fetch emitted index_get on the array VALUE;
the emitter realizes that as a whole-array spill to a stack temp + GEP,
per iteration — O(N^2) bytes copied per loop (and pre-0109 it also grew
the stack per iteration, segfaulting a [4096]s64 loop).

When the iterable is an array with addressable storage (and not deref'd
from a pointer, whose identifier alloca holds the pointer rather than
the array), the fetch is now index_gep on the storage + one element
load. Storage-less arrays keep the index_get fallback. The loaded
element remains a copy — mutating the capture does not write back.

Regression: examples/0048-basic-for-array-large.sx (sum over 4096
elements + by-value copy-guard).

examples/expected/0048-basic-for-array-large.stdout

@@ -0,0 +1,2 @@
+sum=8386560
+copy-guard: 10 20 30