ffi issue-0050: monomorphizeFunction leaks pack-fn state — xfail lock-in

A generic fn (with `$T: Type` type params) called from inside a
pack-fn mono inherits the outer pack maps during its OWN body
lowering. Same root cause as issue-0048 — the lowering helper
doesn't save/null `pack_arg_nodes` / `pack_param_count` /
`pack_arg_types` — but on the generic-mono path
(`monomorphizeFunction`, ~line 8718) rather than
`lazyLowerFunction`.

`examples/175-generic-fn-pack-state-leak.sx` calls
`build(args: []Type, $ret: Type)` from a four-shape pack-fn. The
expected output is `len=0 / 1 / 2 / 4`; today's run reports
`len=0` for every shape because `build__void` was first
monomorphised under `probe()`'s mono (N=0) and `args.len` got
constant-folded to 0 inside the cached body. The next commit
adds the same isolation pattern to `monomorphizeFunction`.

Step 5 of the FFI plan (generic `Into(Block)` impl) needs the
`build_block_convert(args: []Type, $ret: Type) -> string` builder,
which trips this leak directly.

tests/expected/175-generic-fn-pack-state-leak.exit

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

tests/expected/175-generic-fn-pack-state-leak.txt

@@ -0,0 +1,6 @@
+0: len=0
+1: len=1
+2: len=2
+4: len=4
+--- build done ---
+rt