fix(rt): handle zero-sized function types in decode and ZST recognition (#813)

## Summary

Fix two issues preventing correct handling of zero-sized function item
types (e.g., non-capturing closures):

1. **`#zeroSizedType` did not recognize `typeInfoFunType`** (`types.md`)
— Function items are inherently zero-sized (`#elemSize` already returns
0), but `#zeroSizedType` returned `false`, preventing `rvalueRef` from
materializing uninitialized closure locals.

2. **`#decodeConstant` had no rule for `typeInfoFunType`** (`data.md`)
— Zero-sized function constants fell through to the `thunk` wrapper
instead of decoding to `Aggregate(variantIdx(0), .List)`.

## Evidence

`closure-staged.rs` with `terminate-on-thunk`:

| | Steps | Result |
|---|---|---|
| Before | 44 | `thunk(#decodeConstant(constantKindZeroSized, _,
typeInfoFunType(_)))` |
| After | 266 | `#EndProgram` |

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Daniel Cumming <124537596+dkcumming@users.noreply.github.com>

Author: 3 people

kmir/src/kmir/kdist/mir-semantics/rt/data.md

@@ -1921,6 +1921,9 @@ Zero-sized types can be decoded trivially into their respective representation.
   // zero-sized array
   rule <k> #decodeConstant(constantKindZeroSized, _TY, typeInfoArrayType(_, _))
         => Range(.List) ... </k>
+  // zero-sized function item (e.g., closures without captures)
+  rule <k> #decodeConstant(constantKindZeroSized, _TY, typeInfoFunType(_))
+        => Aggregate(variantIdx(0), .List) ... </k>
 ```
 
 Allocated constants of reference type with a single provenance map entry are decoded as references

kmir/src/kmir/kdist/mir-semantics/rt/types.md

@@ -348,8 +348,9 @@ Slices, `str`s  and dynamic types require it, and any `Ty` that `is_sized` does
   rule #zeroSizedType(typeInfoTupleType(.Tys, _)) => true
   rule #zeroSizedType(typeInfoStructType(_, _, .Tys, _)) => true
   rule #zeroSizedType(typeInfoVoidType) => true
-  // FIXME: Only unit tuples, empty structs, and void are recognized here; other
-  // zero-sized types (e.g. single-variant enums, function or closure items,
+  rule #zeroSizedType(typeInfoFunType(_)) => true
+  // FIXME: Only unit tuples, empty structs, void, and function items are
+  // recognized here; other zero-sized types (e.g. single-variant enums,
   // newtype wrappers around ZSTs) still fall through because we do not consult
   // the layout metadata yet. Update once we rely on machineSize(0).
   rule #zeroSizedType(_) => false [owise]

kmir/src/tests/integration/test_integration.py

@@ -41,6 +41,9 @@
     'iter-eq-copied-take-dereftruncate': ['repro'],
     'spl-multisig-iter-eq-copied-next': ['repro'],
 }
+PROVE_TERMINATE_ON_THUNK = [
+    'closure-staged',
+]
 PROVE_SHOW_SPECS = [
     'local-raw-fail',
     'interior-mut-fail',
@@ -87,7 +90,8 @@ def test_prove(rs_file: Path, kmir: KMIR, update_expected_output: bool) -> None:
     if update_expected_output and not should_show:
         pytest.skip()
 
-    prove_opts = ProveOpts(rs_file, smir=is_smir)
+    should_terminate_on_thunk = rs_file.stem in PROVE_TERMINATE_ON_THUNK
+    prove_opts = ProveOpts(rs_file, smir=is_smir, terminate_on_thunk=should_terminate_on_thunk)
     printer = PrettyPrinter(kmir.definition)
     cterm_show = CTermShow(printer.print)