issue-0037 fixed: ptr↔int conversion in coerceToType / bitcast emit

109/109 regression tests pass; chess Android + iOS-sim still
build clean.

Root cause: sx's `xx <ptr>` cast targeting an integer type
(common pattern: `xx u64 = xx @some_global`) lowered to a no-op
because `coerceToType` had branches for int↔float and same-kind
widen/narrow, but nothing for pointer↔integer. The cast left the
value as a pointer Ref, and `emitInst`'s `.ret` arm tried to
coerce a `ptr` value to an `i64` slot — coerceArg had no
ptr↔int branch either, fell through to undef.

Why it worked in main but failed in helpers: an
`alloca u64`+`store ptr @g, alloca`+`load i64, alloca` sequence
preserves the address bits as raw memory, so the
"store-then-load through an alloca" workaround happened to do
the right thing without a real cast. A `ret i64 <ptr>` has no
such intermediate slot and triggers an LLVM type mismatch.

Fix layered into two existing IR opcodes:

  lower.zig (coerceToType):
    new branch — when src and dst types are ptr↔int, emit a
    `bitcast` IR opcode with the right from/to. Mirrors how
    int↔float emits `.int_to_float` / `.float_to_int`.

  emit_llvm.zig (.bitcast arm):
    dispatch ptr→int to `LLVMBuildPtrToInt` (+ trunc/zext if the
    target int width != 64), int→ptr to `LLVMBuildIntToPtr`. The
    "real bitcast" path stays for same-kind type punning.
    Modern LLVM's BuildBitCast rejects ptr↔int directly, hence
    the dispatch.

The fix also closes a quiet behavior gap that affected non-`#foreign`
globals (any `xx @<global>` from a helper fn). Surfaced while
investigating issue-0037; verified independently with a
non-`#foreign` sx-side global of type `s64`.

File mechanics: issue-0037 promoted to a focused feature example
per CLAUDE.md's resolution flow:
  examples/issue-0037.sx        -> examples/102-foreign-global-from-helper.sx
  tests/expected/issue-0037.{txt,exit} -> tests/expected/102-foreign-global-from-helper.{txt,exit}

ffi-objc-call-03 + ffi-objc-call-06 IR snapshots updated to
reflect the ptr→int store-via-ptrtoint shape that's now correct
at the LLVM-IR level (same bits in memory, but properly typed).

tests/expected/ffi-objc-call-03-selector-sharing.ir

@@ -107,7 +107,8 @@ entry:
   store { ptr, i64 } undef, ptr %allocaN, align 8
   %loadN = load ptr, ptr %allocaN, align 8
   %gep = getelementptr inbounds { ptr, i64 }, ptr %allocaN, i32 0, i32 0
-  store ptr %loadN, ptr %gep, align 8
+  %pti = ptrtoint ptr %loadN to i64
+  store i64 %pti, ptr %gep, align 8
   %loadN = load i64, ptr %alloca, align 8
   %gepN = getelementptr inbounds { ptr, i64 }, ptr %allocaN, i32 0, i32 1
   store i64 %loadN, ptr %gepN, align 8