ffi issue-0045 fix: inline-return slot for comptime-call bodies

`lowerComptimeCall` now scans the body for `return` statements
via `fnBodyHasReturn`. When found, it allocates a stack slot
typed to the fn's return type and installs it as
`self.inline_return_target` before lowering the body.

`lowerReturn` checks `inline_return_target` first:
- If set, it stores the coerced return value into the slot,
  drains pending defers, sets `block_terminated = true`, and
  returns without emitting a `ret` into the caller's basic
  block.
- Otherwise it emits the standard `ret` as before.

After the body lowers, the inliner either returns the
tail-expression value (existing fast path — bodies with no
`return` skip the slot entirely) or loads the slot when
`block_terminated` is set.

Why the bug was invisible until now: `format`/`print` and
every other stdlib comptime fn use arrow form (`=> expr`) or
`#insert`-only bodies — no `return` statement, no path through
`lowerReturn`. Step 1.b of the pack feature made `..$args`
parseable; the natural smoke test
`foo :: (..$args) -> s64 { return 42; }` was the first
comptime-fn body to take the `return`-with-trailing-statements
path, surfacing the LLVM verifier crash.

`examples/issue-0045.sx` flips from the lock-in failure to
`42`. 194/194 example tests + `zig build test` green.

issues/0045-pack-fn-call-llvm-verifier-failure.md

@@ -1,3 +1,9 @@
+**FIXED.** `lowerComptimeCall` now allocates a result slot when
+the body contains a `return` statement and reroutes
+`lowerReturn` to store into it instead of emitting `ret` into the
+caller's basic block. Regression test:
+[examples/issue-0045.sx](../examples/issue-0045.sx).
+
 # Symptom
 
 Calling a fn declared with `..$args` (variadic heterogeneous type