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-core/src/ast.rs

@@ -129,8 +129,26 @@ pub struct TypeDef {
     /// Optional source-level documentation string.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub doc: Option<String>,
+    /// Iter 18e: opt-in `(drop-iterative)` annotation. When `true`,
+    /// codegen emits `drop_<m>_<T>` with an iterative worklist body
+    /// instead of the recursive cascade — chosen by the LLM-author when
+    /// the type is expected to form long chains (millions of cells)
+    /// that would overflow the C stack on free.
+    ///
+    /// Serialised as `"drop-iterative": true` (kebab-case) when set;
+    /// the field is omitted when `false` so canonical-JSON hashes of
+    /// every pre-18e fixture remain bit-stable. See the regression
+    /// test `iter18e_drop_iterative_default_preserves_hashes` in
+    /// [`crate::hash`].
+    #[serde(
+        default,
+        rename = "drop-iterative",
+        skip_serializing_if = "is_false"
+    )]
+    pub drop_iterative: bool,
 }
 
+
 /// A single constructor of a [`TypeDef`].
 ///
 /// `fields` is the constructor's positional argument list as types;

crates/ailang-core/src/desugar.rs

@@ -1959,6 +1959,7 @@ mod tests {
                 ],
             }],
             doc: None,
+            drop_iterative: false,
         };
         let letrec = Term::LetRec {
             name: "helper".into(),