fix(lower): closure literals compose with bare function-type slots (issue 0060)

A closure's underlying function carries a hidden env arg that a bare (T)->U slot
doesn't pass, so a closure flowing into a bare function-type slot dropped the
env — the first user arg landed in the env slot and the rest read garbage
(apply(closure((x)->s64 { x*2 })) returned 192 instead of 10; non-failable too).

- createClosureToBareFnAdapter: a capture-free closure into a bare (T)->U slot is
  bridged by a generated adapter carrying the bare ABI (forwards a null env);
  lowerLambda returns its func_ref. Rejected (no silent miscompile): a capturing
  closure into a bare slot (env has nowhere to live) and a failable closure into
  a non-failable slot (the ERR E5.1 FFI-boundary rule).
- Arrow-body failable closures (-> (T,!) => expr) now wrap the bare success value
  into {value, 0} via lowerFailableSuccessReturn (the implicit return previously
  returned a malformed tuple → caught value read as 0).

The isLambda .bang parser fix (failable closure literals parse) already landed in
485b4fa. Regressions: examples/0309-closures-literal-as-bare-fn-param (non-
failable, block + arrow, called in callee) + 1039-errors-failable-closure-literal
(failable, block + arrow, direct + Closure(...) param). Resolves issue 0060
(remaining E5.1 follow-ups noted in the .md). Suite: 328 passed.

examples/expected/0309-closures-literal-as-bare-fn-param.exit

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

examples/expected/0309-closures-literal-as-bare-fn-param.stderr

@@ -0,0 +1 @@
+

examples/expected/0309-closures-literal-as-bare-fn-param.stdout

@@ -0,0 +1,3 @@
+block=10
+arrow=10
+twice=7

examples/expected/1039-errors-failable-closure-literal.exit

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

examples/expected/1039-errors-failable-closure-literal.stderr

@@ -0,0 +1 @@
+

examples/expected/1039-errors-failable-closure-literal.stdout

@@ -0,0 +1,2 @@
+10 99 7 41
+param ok=10 err=-1