ERR/E5.1: bare failable fn-type param resolution + non-failable->failable widening

Two more E5.1 composition pieces:
- inferExprType .call: a callee that's a local variable of bare  type
  () now resolves to its declared return type (only
  was handled before), so  /  on the call see the failable result
  instead of .
- createClosureToBareFnAdapter now widens: when a NON-failable closure literal
  flows into a failable bare slot (∅ ⊆ slot set, success type matches), the
  adapter wraps the value into the slot's  tuple via
  lowerFailableSuccessReturn — previously rejected. The failable->non-failable
  and capturing->bare crossings stay rejected.

Adapter generation fires for closure LITERALS flowing into a bare-fn slot; a
pre-bound closure VARIABLE into a bare-fn slot is a separate coercion-site path,
still unhandled (noted in CHECKPOINT-ERR). Regression:
examples/1040-errors-failable-closure-composition. Suite: 329 passed.

examples/1040-errors-failable-closure-composition.sx

@@ -0,0 +1,34 @@
+// Failable closure composition (ERR E5.1): a closure LITERAL passed as a
+// function-type argument and called inside the callee. Covers a bare failable
+// fn-type param (`cb: (T) -> (U, !)`), the idiomatic `Closure(...)` param
+// (try-propagated), and ∅-widening of a NON-failable closure literal into a
+// failable slot (the generated adapter wraps the value into `{value, 0}`).
+//
+// NOTE: the adapter is generated when the closure LITERAL flows directly into
+// the bare-fn slot. Passing a pre-bound closure *variable* into a bare-fn slot
+// is a separate coercion-site path, not yet handled — see CHECKPOINT-ERR.
+
+#import "modules/std.sx";
+
+E :: error { Neg }
+
+bare  :: (cb: (s64) -> (s64, !E), n: s64) -> s64 { return cb(n) catch e -1; }
+chain :: (cb: Closure(s64) -> (s64, !E), n: s64) -> (s64, !E) { return try cb(n); }
+
+dbl :: (x: s64) -> (s64, !E) { if x < 0 { raise error.Neg; } return x * 2; }
+
+main :: () -> s32 {
+    // failable closure literal through a bare fn-type param (matching ABI)
+    print("bare ok={} err={}\n",
+        bare(closure((x: s64) -> (s64, !E) { if x < 0 { raise error.Neg; } return x * 2; }), 5),
+        bare(closure((x: s64) -> (s64, !E) => x * 2), -1));                 // ok=10; err: arrow never raises → cb(-1) = -2
+
+    // Closure(...) param, try-propagated, then caught at the call site
+    print("chain ok={} err={}\n",
+        chain(closure((x: s64) -> (s64, !E) => x + 6), 4) catch e 0,        // 10
+        chain(closure((x: s64) -> (s64, !E) { raise error.Neg; }), 1) catch e 0); // 0
+
+    // NON-failable closure literal widened into the failable bare slot
+    print("widen={}\n", bare(closure((x: s64) -> s64 => x + 1), 9));        // 10
+    return 0;
+}

examples/expected/1040-errors-failable-closure-composition.exit

@@ -0,0 +1 @@
+0

examples/expected/1040-errors-failable-closure-composition.stderr

@@ -0,0 +1 @@
+

examples/expected/1040-errors-failable-closure-composition.stdout

@@ -0,0 +1,3 @@
+bare ok=10 err=-2
+chain ok=10 err=0
+widen=10