feat(mods): add screen render visibility hook

docs/rfcs/0002-screen-render-visible.md

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+# RFC 0002 — Let mods hide an active screen state from the main render
+
+## Status
+
+Proposed. Engine: `StateStack.lua`, `Game.lua`. Tests:
+`screen_render_visible.lua`.
+
+## Motivation
+
+A mod can render a native menu on a companion display through
+`render.compose`, but it cannot remove that menu from the main display without
+also popping it. Popping transfers update and input ownership and forces the
+mod to reimplement native menu behavior.
+
+## The decision it extends
+
+No prior D-number. Extends the render-hook plan in `docs/modding.md` and the
+state-stack rendering contract in `docs/architecture.md`.
+
+## The exact API delta
+
+Backward-compatible, additive-only.
+
+### `screen.render_visible`
+
+New hook called with `(state) -> boolean` through the public wrapper signature
+`(next, state)`. Its vanilla result is `true`.
+
+Returning `false` excludes the state from the main draw, from opaque-base
+selection and from palette-zone ownership. It does not remove the state or
+change update, input, push or pop behavior. The call sites are
+`StateStack:visibleBase`, `StateStack:draw` and the equivalent draw and palette
+walks in `Game:draw`.
+
+The hook is guarded by `Runtime.wantsHook`, so the no-subscriber path allocates
+nothing. It is a pure render predicate and may be evaluated more than once per
+frame.
+
+## Migration note for existing mods
+
+**Nothing.** With no subscriber every state remains visible, and the existing
+state-stack, event and hook behavior is unchanged.
+
+## Parity tests
+
+- **No-mod:** the topmost opaque state still owns drawing and palette zones,
+  and `Runtime.wantsHook("screen.render_visible")` stays false.
+- **Mod-API:** a fixture mod registers through `mod.hooks:wrap`, hides one
+  opaque state and proves the state beneath draws and owns the palette while
+  the hidden state remains topmost and continues updating.
+
+## Deprecation etiquette
+
+Nothing deprecated. This is one additive hook with a `true` vanilla default.