docs: record that text and 9-slice parts are not drawn

[Naruto]

Why

ss_internal_player.cpp has said this in a comment since the batch dispatch was written:

// DrawBatchKind_Text / Nines / Mask: not yet implemented.

…but docs/{en,ja}/limitations.md never did, so an author had no way to find out short of trying it and seeing nothing appear. (Mask in that list is fine — pure masks feed the coverage bitmap and are drawn elsewhere. Text and Nines are genuinely unimplemented.)

This came out of an ecosystem-wide audit: all five official players are in the same position, and none but Ren'Py documented it.

What changed

A Not Yet Implemented section, placed first — matching where Ren'Py puts its equivalent.

9-slice is the surprising half. The runtime builds the entire grid — margins, fill mode, repeats — into dedicated Nines SoA buffers (ss_runtime_get_nines_vertices and friends), and local_vertices carries only the part's plain bounding quad. So a player that does not read those buffers draws nothing at all, not a stretched cell. "The runtime builds it, so it just works" is not true here.

Both part types keep their place in the draw order, so an animation that uses them plays with those parts missing rather than failing — worth saying, because it means the symptom is a silent gap rather than a crash.

Scope

Documentation only, both locales. No behaviour change.

.venv/bin/mkdocs build --strict passes.