fix: 18g.2 — let-binder drop for Own-returning App

18c.3's is_rc_heap_allocated returned false for any Term::App
shape; the doc-comment explicitly deferred the owned-returning-call
case to 'later iters tied to (own) ret-mode contracts'. We have
those contracts (Iter 18a). With this iter the predicate widens to
recognise Term::App whose callee carries ret_mode=Own; the let-
scope close emits a drop call, routing through the per-type drop
fn derived from the call's return type so the cascade walks ptr
children correctly.

Borrow- and Implicit-returning calls are still not trackable —
they don't carry the static caller-owns-the-return signal.

drop_symbol_for_binder gains an App arm that synthesises the
return type and resolves drop_<m>_<T> from it (same shape as the
existing Ctor arm), with cross-module qualification through
import_map. emit_inlined_partial_drop now defaults to shallow
ailang_rc_dec when value is not Term::Ctor — that path is the
dynamic-tag partial-drop debt 18d.4 already documents (the
runtime tag of a let-binder whose value is Term::App is not
statically known, so per-field partial-drop emission is not
usable; closing this remaining leak path requires a tag-
conditional helper, queued as future work).

Red: alloc_rc_let_binder_for_owned_returning_app_drops_at_scope_close
on examples/rc_let_owned_app_leak — pre-fix live=3 (TNode + 2
TLeaf), post-fix live=0.

End-to-end: bench_latency_explicit under --alloc=rc now reports
allocs=11068575 frees=11068575 live=0. The depth-19 Tree cache
+ all 10M per-op IntList cells deallocate cleanly. Decision
10's prompt-deallocation property now holds end-to-end on the
canonical bench fixture.

examples/rc_let_owned_app_leak.ail.json

@@ -0,0 +1 @@
+{"defs":[{"ctors":[{"fields":[],"name":"TLeaf"},{"fields":[{"k":"con","name":"Int"},{"k":"con","name":"Tree"},{"k":"con","name":"Tree"}],"name":"TNode"}],"kind":"type","name":"Tree"},{"body":{"cond":{"args":[{"name":"d","t":"var"},{"lit":{"kind":"int","value":0},"t":"lit"}],"fn":{"name":"==","t":"var"},"t":"app"},"else":{"args":[{"lit":{"kind":"int","value":1},"t":"lit"},{"args":[{"args":[{"name":"d","t":"var"},{"lit":{"kind":"int","value":1},"t":"lit"}],"fn":{"name":"-","t":"var"},"t":"app"}],"fn":{"name":"build","t":"var"},"t":"app"},{"args":[{"args":[{"name":"d","t":"var"},{"lit":{"kind":"int","value":1},"t":"lit"}],"fn":{"name":"-","t":"var"},"t":"app"}],"fn":{"name":"build","t":"var"},"t":"app"}],"ctor":"TNode","t":"ctor","type":"Tree"},"t":"if","then":{"args":[],"ctor":"TLeaf","t":"ctor","type":"Tree"}},"kind":"fn","name":"build","params":["d"],"type":{"effects":[],"k":"fn","params":[{"k":"con","name":"Int"}],"ret":{"k":"con","name":"Tree"},"ret_mode":"own"}},{"body":{"arms":[{"body":{"lit":{"kind":"int","value":0},"t":"lit"},"pat":{"ctor":"TLeaf","fields":[],"p":"ctor"}},{"body":{"lit":{"kind":"int","value":1},"t":"lit"},"pat":{"ctor":"TNode","fields":[{"name":"v","p":"var"},{"name":"l","p":"var"},{"name":"r","p":"var"}],"p":"ctor"}}],"scrutinee":{"name":"t","t":"var"},"t":"match"},"kind":"fn","name":"pin","params":["t"],"type":{"effects":[],"k":"fn","param_modes":["borrow"],"params":[{"k":"con","name":"Tree"}],"ret":{"k":"con","name":"Int"}}},{"body":{"body":{"args":[{"args":[{"name":"t","t":"var"}],"fn":{"name":"pin","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"name":"t","t":"let","value":{"args":[{"lit":{"kind":"int","value":1},"t":"lit"}],"fn":{"name":"build","t":"var"},"t":"app"}},"kind":"fn","name":"main","params":[],"type":{"effects":["IO"],"k":"fn","params":[],"ret":{"k":"con","name":"Unit"}}}],"imports":[],"name":"rc_let_owned_app_leak","schema":"ailang/v0"}

examples/rc_let_owned_app_leak.ailx

@@ -0,0 +1,53 @@
+; Iter 18g.2 RED-test fixture: a let-binder whose value is the
+; result of an Own-returning fn call. 18c.3's
+; `is_rc_heap_allocated` predicate returns `false` for any
+; `Term::App` shape (its doc-comment explicitly defers this case
+; to "later iters tied to (own) ret-mode contracts"); we now have
+; those contracts (Iter 18a), and the let-scope close should drop
+; the binder.
+;
+; Surfaced by 18f.2 / 18g.1: `bench_latency_explicit`'s
+; `(let t (app build_tree 19) ...)` leaks the entire depth-19
+; Tree (~1M cells) because `t` is not trackable. This minimal
+; repro uses a 2-cell depth-1 tree.
+;
+; Pre-fix:  live = 3 (TNode + 2 TLeaf children — the depth-1
+;           tree).
+; Post-fix: live = 0.
+
+(module rc_let_owned_app_leak
+
+  (data Tree
+    (ctor TLeaf)
+    (ctor TNode (con Int) (con Tree) (con Tree)))
+
+  (fn build
+    (type
+      (fn-type
+        (params (con Int))
+        (ret (own (con Tree)))))
+    (params d)
+    (body
+      (if (app == d 0)
+        (term-ctor Tree TLeaf)
+        (term-ctor Tree TNode 1
+          (app build (app - d 1))
+          (app build (app - d 1))))))
+
+  (fn pin
+    (type
+      (fn-type
+        (params (borrow (con Tree)))
+        (ret (con Int))))
+    (params t)
+    (body
+      (match t
+        (case (pat-ctor TLeaf) 0)
+        (case (pat-ctor TNode v l r) 1))))
+
+  (fn main
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (let t (app build 1)
+        (do io/print_int (app pin t))))))