feat(host/hooks): per-app prep/undo commands (M2b — Sunshine prep-cmd parity)

`prep: [{"do": …, "undo": …}]` arrays on GameStream apps.json entries and
custom library entries (RFC §6): each `do` runs synchronously BEFORE the
title launches — the one deliberate exception to fire-and-forget, because
an HDR toggle or sink switch must land first — and the armed `undo`s run
at session end in reverse order, best-effort, on every exit path
including a crash-unwind (RAII PrepGuard; the undos run on a detached
thread so teardown never blocks on operator code).

- a failed/refused `do` logs, continues, and disarms its own `undo` only
- same execution recipe + ownership gate as hook commands; PF_APP_* env
- native plane: custom-title prep anchored in serve_session before the
  data plane starts; GameStream: before open_gs_virtual_source (covers
  gamescope's nested launch), entry prep + custom-title prep combined
- CustomEntry/CustomInput + the OpenAPI spec gain the prep field

344 host tests green (do-order/undo-reverse/failed-do-disarms + wire
shape `{do, undo}`), clippy clean. On-glass with a real client session
owed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/native.rs

@@ -1110,6 +1110,17 @@ async fn serve_session(
             }
         }
     });
+    // Per-title prep steps (RFC §6) for a launched CUSTOM library title: run synchronously
+    // before the data plane starts (so before the display opens and the title spawns); the
+    // guard's drop — any serve_session exit — runs the undos in reverse, best-effort.
+    // `block_in_place`: prep is blocking operator code and this is a multi-thread runtime;
+    // the closure only runs when the title actually has prep steps.
+    let _prep = hello.launch.as_deref().and_then(|id| {
+        let cmds = crate::library::prep_for(id);
+        let env = [("PF_APP_ID".to_string(), id.to_string())];
+        (!cmds.is_empty())
+            .then(|| tokio::task::block_in_place(|| crate::hooks::run_prep(&cmds, &env)))
+    });
     let bitrate_kbps = welcome.bitrate_kbps; // resolved encoder bitrate (Hello clamped, or default)
                                              // "Automatic" request: the resolved rate is a host default — for PyroWave a per-mode
                                              // bpp pin the data plane re-resolves on a mid-stream mode switch.