ffi M5.A.next.3a.C: $args[$i] in fn-pointer type literals

Adds `resolveFunctionTypeWithBindings` so `function_type_expr`
in a binding-aware context — local var annotations, return
types, nested type expressions — recursively resolves through
the active pack bindings. Without this, the fall-through to
`type_bridge.resolveAstType` lost pack context and the new
`pack_index_type_expr` arm spammed the "outside pack-aware
context" diagnostic (the function still worked by accident
thanks to the `.s64` fallback).

Plumbing:
- `resolveTypeWithBindings` adds a `function_type_expr` case
  in both the bindings-active branch and the fallthrough
  switch (the same shape as `closure_type_expr`).
- `resolveFunctionTypeWithBindings` recursively resolves each
  param + return type with bindings, then calls
  `functionTypeCC` with the AST's calling convention.

`examples/167-pack-type-fnptr.sx` exercises the pattern step
5's trampoline needs:
  fp : (*void, $args[0]) -> $args[1] = double_s64;
  return fp(null, args[0]);
Output: 14 (= 7*2 via the typed fn-pointer).

207/207 example tests + `zig build test` green.

tests/expected/167-pack-type-fnptr.exit

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

tests/expected/167-pack-type-fnptr.txt

@@ -0,0 +1 @@
+14