ffi issue-0050: monomorphizeFunction isolates pack-fn mono state

Adds the same save+null+defer-restore block at the top of
`monomorphizeFunction` that landed in `lazyLowerFunction` for
issue-0048. The outer pack-fn's `pack_arg_nodes` /
`pack_param_count` / `pack_arg_types` / `inline_return_target`
are now suppressed for the duration of the generic mono's body
lowering and restored on exit.

`examples/175-generic-fn-pack-state-leak.sx` flips green
(len=0/1/2/4 across the four pack shapes); suite stays at
215/215.

src/ir/lower.zig

@@ -8730,6 +8730,29 @@ pub const Lowering = struct {
         const saved_defer_base = self.func_defer_base;
         const saved_block_terminated = self.block_terminated;
         const saved_target = self.target_type;
+        // Pack-fn mono state is lexical to the pack-fn body. A generic
+        // function called from inside a pack-fn mono (e.g.
+        // `build(args: []Type, $ret: Type)` invoked from
+        // `probe(..$args) { build($args, void) }`) must not inherit the
+        // caller's pack maps — `lowerFieldAccess`'s `<pack_name>.len`
+        // intercept would otherwise constant-fold the callee's
+        // same-named param to whichever shape triggered the first mono
+        // and bake the wrong arity into the cached IR. Same shape of
+        // fix as `lazyLowerFunction` (issue-0048, commit 0ede097).
+        const saved_pan = self.pack_arg_nodes;
+        const saved_ppc = self.pack_param_count;
+        const saved_pat = self.pack_arg_types;
+        const saved_iri = self.inline_return_target;
+        self.pack_arg_nodes = null;
+        self.pack_param_count = null;
+        self.pack_arg_types = null;
+        self.inline_return_target = null;
+        defer {
+            self.pack_arg_nodes = saved_pan;
+            self.pack_param_count = saved_ppc;
+            self.pack_arg_types = saved_pat;
+            self.inline_return_target = saved_iri;
+        }
         self.func_defer_base = self.defer_stack.items.len;
         self.block_terminated = false;