fix(ir): store to module-global array element targets live storage (issue 0079)

A store to a module-global array element (`g[i] = v`) was silently dropped:
a subsequent `g[i]` read the array's initializer, not `v`. Constant index,
variable index, and cross-function stores were all affected, in both `sx run`
and `sx build`. Global scalars and local arrays were fine.

Root cause: `Lowering.lowerExprAsPtr` (the lvalue/address path) handled only
local identifiers. A module-global identifier fell through to the value
fallback `lowerExpr`, which emits `global_get` — loading the whole array by
value. The LLVM backend's `emitIndexGep` then allocas a throwaway temp, copies
the value in, and GEPs into the temp, so the store wrote a discarded copy.

Fix: teach `lowerExprAsPtr`'s identifier arm about globals — emit `global_addr`
(a pointer into the global's live storage), or `global_get` for a pointer-typed
global (mirroring the local pointer case). Route the `address_of(index_expr)`
array base through `lowerExprAsPtr` too so `&g[i]` is likewise an lvalue into
the global. `index_gep` now GEPs directly into the global for const and variable
index, across functions. This also fixes global struct field stores, which
shared the same root cause.

Regression: examples/0136-types-global-array-element-store.sx (const-index,
var-index, cross-function store on a scalar global array; struct-element array
for stride; nested-array global for the recursive lvalue). Fails on the pre-fix
compiler, passes after.

examples/expected/0136-types-global-array-element-store.exit

@@ -0,0 +1 @@
+0

examples/expected/0136-types-global-array-element-store.stderr

@@ -0,0 +1 @@
+

examples/expected/0136-types-global-array-element-store.stdout

@@ -0,0 +1,8 @@
+g[1]=222
+g[k]=333
+g[0]=111
+gp[0]=10,20
+gp[j]=30,40
+grid[1][2]=7
+grid[0][0]=5
+PASS