fix: resolve closure/fn-pointer struct-field call types (issue 0201)

Calling a closure or function-pointer value stored in a struct data field
(`box.run(args)`) typed the call as 'unresolved': value returns marshalled
as garbage, failable fields could not be try/catch-ed. Lowering already
dispatched these (call_closure / call_indirect); only CallResolver.plan
lacked a field-access arm. Add a closure/fn-pointer field arm to plan
(before the instance-method check, mirroring lowering's precedence — a
closure-typed field shadows a same-named method) and extend the lowering
closure-field arm to also handle bare .function fields via call_indirect.

Lock: examples/closures/0315-closures-struct-field-call.sx.

issues/0201-closure-struct-field-call-unresolved.md

@@ -0,0 +1,91 @@
+> **RESOLVED** (2026-06-28). Root cause: `CallResolver.plan`'s `.field_access`
+> branch had no arm for a struct field whose type is a `.closure` / `.function`
+> value, so the call typed as `.unresolved` (the lowering side already dispatched
+> it via `call_closure`/`call_indirect`, but the type used for arg-boxing /
+> `try`/`catch` came from `plan`). Fix: added a closure/fn-pointer field arm in
+> `src/ir/calls.zig` (`plan`), placed before the instance-method check to mirror
+> lowering's precedence (a closure-typed field shadows a same-named method), and
+> extended the lowering closure-field arm in `src/ir/lower/call.zig` to also
+> handle bare `.function` fields (`call_indirect`, ctx-prepend gated on the
+> fn-ptr ABI). Regression test:
+> `examples/closures/0315-closures-struct-field-call.sx` (closure field with
+> args, failable field via `*self` receiver — success + error, and a bare
+> fn-pointer field). Suite 852/0.
+
+# 0201 — calling a closure stored in a struct field types as `unresolved`
+
+**Symptom** — Calling a closure value held in a **struct data field**
+(`box.run()` where `run: Closure(...) -> R`) does not resolve the call's
+return type: the result types as `unresolved`. For a value-returning closure
+this silently produces garbage (the result is never marshaled); for a failable
+closure (`Closure() -> (T, !)`) `try`/`catch` reject the call with "`catch`
+requires a failable expression; operand has type 'unresolved'".
+
+Observed vs expected:
+- `b.run()` where `run: Closure() -> i64` prints a garbage pointer-ish integer
+  (e.g. `4313325408`) instead of the closure's actual return value `7`.
+- The IDENTICAL closure bound to a **local variable** (`f := () => {7}; f()`)
+  works and prints `7`.
+- A **void**-returning closure field (`run: Closure() -> void; b.run()`) works
+  for its side effects (no result to marshal), which is why `std/io.sx`'s
+  `ThunkBox { run: Closure() -> void }` is unaffected.
+
+**Scope / impact** — Pre-existing, independent of the PLAN-IO-UNIFY Phase 3
+capture-typing fix (it reproduces at top level with no closures-capturing-
+closures and no nesting). Does **NOT** block Phase 3: the async layer routes all
+generic-ness through a captured worker + a void completion closure field, both of
+which work. Worth fixing because the value-return case is silent corruption.
+
+**Reproduction** (standalone, only needs the prelude):
+
+```sx
+#import "modules/std.sx";
+Box :: struct { run: Closure() -> i64; }
+main :: () -> i64 {
+    b : Box = ---;
+    b.run = () => { 7 };
+    print("{}\n", b.run());   // prints garbage; expected 7
+    return 0;
+}
+```
+
+Failable variant (the shape that surfaced it):
+
+```sx
+#import "modules/std.sx";
+Box :: struct { run: Closure() -> (i64, !); }
+main :: () -> i64 {
+    b : Box = ---;
+    b.run = () -> (i64, !) => { 7 };
+    r := b.run() catch { return -1; };   // error: catch requires a failable
+    print("{}\n", r);                     //        expression; operand 'unresolved'
+    return 0;
+}
+```
+
+**Investigation prompt** — The call-type resolver `CallResolver.plan` in
+[src/ir/calls.zig](../src/ir/calls.zig) has a `field_access` callee branch
+(~line 230 onward) that handles protocol dispatch, runtime-class instance
+methods, `StructName.method` instance methods, and free-fn UFCS — but has **no
+arm for a struct field whose type is a `.closure` (or `.function`) value**. When
+none of those match it falls through to `.unresolved` (e.g. line ~315 / the tail
+return). Compare the BARE-identifier path (~lines 211–227) which already handles
+`ti == .closure → ti.closure.ret` / `ti == .function → ti.function.ret` for a
+local binding — the field-access path needs the equivalent: resolve the
+receiver's struct type, look up the named field, and if the field's type is a
+closure/function, return its `.ret` with the right call kind (an indirect/closure
+call on the loaded field value).
+
+The fix likely needs a new `CallPlan.kind` (or reuse of the closure-call kind)
+for "call a closure loaded from a struct field", and the lowering side
+(`lowerCall` field-access path in [src/ir/lower/expr.zig](../src/ir/lower/expr.zig))
+must load the field then perform an indirect closure call (env + fn-ptr), the
+same machinery a local closure-variable call uses. Mind the failable case: once
+the return type resolves to a `(T, !)` tuple, `errorChannelOf` and the
+`try`/`catch` paths work automatically (verified: a local failable closure call
+already does).
+
+**Verification** — run both repros above; expect `7` from the first and `7`
+(success) / `-1` (error) from the failable variant, with no `unresolved`
+diagnostic. Add a regression example under `examples/closures/` (value + failable
+field-call) once fixed.