Iter 18e: (drop-iterative) annotation + worklist allocator

Closes the 18-arc's stack-recursion limit. Recursive drop
cascades from 18c.4 overflow on long ADT chains (Linux's 8 MB
default stack maxes out around 1M cells of List). The new
opt-in (drop-iterative) annotation on a Def::Type switches the
synthesised drop_<m>_<T> body from recursive to iterative-with-
explicit-worklist for that type.

Schema:
- TypeDef.drop_iterative: bool. Default false; serde-skip
  when false so existing fixtures' canonical JSON hashes stay
  stable.
- Form-A: (drop-iterative) clause inside (data T ...).

Worklist runtime (4 new ABI symbols in runtime/rc.c):
- ailang_drop_worklist_new(initial_capacity)
- ailang_drop_worklist_push(wl, ptr)
- ailang_drop_worklist_pop(wl) -> ptr
- ailang_drop_worklist_free(wl)

Heap stretchy buffer, doubling on overflow, null-filtering on
push. Lean 4 / Roc precedent documented in the runtime; the
slot-repurposing strategy was considered and rejected because
not every box has a free pointer-typed slot to thread the
worklist through (Cons head is i64, slot 1 is ptr but it's
the field we're following — no free slot).

Codegen (emit_iterative_drop_fn_for_type): for a
drop_iterative type, drop_<m>_<T>(ptr %p) emits a worklist
loop. Fields of the SAME annotated type push onto the
worklist (mono-typed); fields of DIFFERENT types call their
own drop fn directly (recursive on those, but only if THEY
are themselves recursive — i.e. one level of cascade jump
maximum). Mono-typed-worklist is sound for the deep-self-
recursion case the iter targets (List of List of T just
needs the spine flattened).

Tests:
- examples/rc_drop_iterative_long_list — 1M-cell List of Int
  with (drop-iterative) annotation.
- alloc_rc_drop_iterative_handles_million_cell_list E2E —
  builds + runs under --alloc=rc, asserts clean exit. With
  annotation: exits 0. Without annotation (control): SIGSEGV
  at exit code 139 (verified by hand). Worklist is load-
  bearing.
- iter18e_drop_iterative_emits_worklist_body_no_self_recursion
  IR-shape: worklist body has br to loop_head AND no direct
  recursive call into drop_<m>_<T>.
- iter18e_no_annotation_keeps_recursive_drop_body — control:
  unannotated variant still emits the 18c.4 recursive shape.
- 3 surface parse-tests for the annotation round-trip.

Test deltas: e2e 58 -> 61 (+3), surface 18 -> 21 (+3). All
other buckets unchanged. cargo test --workspace green.

Known debt (deliberate):
- Mono-typed worklist: cross-type drop-iterative fields call
  the other type's drop fn directly. A heterogeneous
  worklist would be more general but adds tag tracking
  complexity for a case (drop-iterative T containing
  drop-iterative T') that's narrower than the deep-self-
  recursion target. Documented in
  emit_iterative_drop_fn_for_type's doc.
- Closure / Type::Var / Type::Forall fields fall back to
  shallow ailang_rc_dec via field_drop_call — same as the
  recursive variant.
- Dynamic-tag partial-drop fallback (head_or_zero epilogue
  shallow dec when moved_slots non-empty) — out of scope per
  brief.

crates/ailang-check/src/lib.rs

@@ -2192,6 +2192,7 @@ mod tests {
                         },
                     ],
                     doc: None,
+                    drop_iterative: false,
                 }),
                 fn_def(
                     "f",
@@ -2242,6 +2243,7 @@ mod tests {
                         },
                     ],
                     doc: None,
+                    drop_iterative: false,
                 }),
                 fn_def(
                     "f",
@@ -2543,6 +2545,7 @@ mod tests {
                 fields: vec![Type::Var { name: "a".into() }],
             }],
             doc: None,
+            drop_iterative: false,
         });
         // fn make :: () -> Box<Int> = MkBox(42)
         let make = fn_def(
@@ -2585,6 +2588,7 @@ mod tests {
                 fields: vec![Type::Var { name: "a".into() }],
             }],
             doc: None,
+            drop_iterative: false,
         });
         let unbox = Def::Fn(FnDef {
             name: "unbox".into(),
@@ -2677,6 +2681,7 @@ mod tests {
                 fields: vec![Type::Var { name: "a".into() }],
             }],
             doc: None,
+            drop_iterative: false,
         });
         let bad = fn_def(
             "bad",
@@ -2763,6 +2768,7 @@ mod tests {
                         },
                     ],
                     doc: None,
+                    drop_iterative: false,
                 }),
                 fn_def(
                     "loop",
@@ -2815,6 +2821,7 @@ mod tests {
                         fields: vec![Type::Var { name: "a".into() }],
                     }],
                     doc: None,
+                    drop_iterative: false,
                 }),
                 Def::Type(TypeDef {
                     name: "Bag".into(),
@@ -2824,6 +2831,7 @@ mod tests {
                         fields: vec![Type::Var { name: "a".into() }],
                     }],
                     doc: None,
+                    drop_iterative: false,
                 }),
             ],
         };
@@ -2953,6 +2961,7 @@ mod tests {
                 vars: vec![],
                 ctors: vec![Ctor { name: "Mk".into(), fields: vec![] }],
                 doc: None,
+                drop_iterative: false,
             })],
         };
         let lib_b = Module {
@@ -2964,6 +2973,7 @@ mod tests {
                 vars: vec![],
                 ctors: vec![Ctor { name: "Mk".into(), fields: vec![] }],
                 doc: None,
+                drop_iterative: false,
             })],
         };
         let consumer = Module {
@@ -3049,6 +3059,7 @@ mod tests {
                     },
                 ],
                 doc: None,
+                drop_iterative: false,
             })],
         };
         // Consumer: builds `Cons 1 (Cons 2 (Nil))` via qualified
@@ -3141,6 +3152,7 @@ mod tests {
                         },
                     ],
                     doc: None,
+                    drop_iterative: false,
                 }),
                 fn_def(
                     "drain",
@@ -3502,6 +3514,7 @@ mod tests {
                 ],
             }],
             doc: None,
+            drop_iterative: false,
         };
         let letrec = Term::LetRec {
             name: "helper".into(),

crates/ailang-check/src/reuse_shape.rs

@@ -565,6 +565,7 @@ mod tests {
                 },
             ],
             doc: None,
+            drop_iterative: false,
         }
     }