fix: capture failable closures called via error-handling exprs

collectCaptures did not descend into catch/try/onfail/raise/multi_assign/
push/comptime/insert/spread/asm nodes, so a free variable referenced only
inside them (e.g. a failable worker called as `worker() catch {…}` in a
nested lambda) was never captured into the env struct — inside the lambda
it resolved against an empty scope and typed as 'unresolved'. Add the
missing traversal arms. The push_stmt arm also closes the noted
'free-var analysis does not descend into a nested push Context {…}' gap.

Unblocks the PLAN-IO-UNIFY Phase 3 async completion closure shape.
Lock: examples/closures/0314-closures-capture-failable-call.sx.

examples/closures/expected/0314-closures-capture-failable-call.exit

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

examples/closures/expected/0314-closures-capture-failable-call.stderr

@@ -0,0 +1 @@
+

examples/closures/expected/0314-closures-capture-failable-call.stdout

@@ -0,0 +1,3 @@
+ok 7
+caught
+try 105