build system + kde: fix stale-cache force-rebuild; bump plasma-wayland-protocols to 1.21.0

Root cause of kf6-kwayland's 'PlasmaWaylandProtocols >= 1.21.0 not found': the
plasma-wayland-protocols recipe (a custom data-only recipe) HARD-CODED
set(PACKAGE_VERSION "1.16.0") in the hand-written ConfigVersionCmake heredoc,
while the package + source are 1.21.0. Bumped it to 1.21.0.

Build-system fix (src/cook/cook_build.rs): the !check_source early-return served
the cached stage for dependency cooks WITHOUT honoring --force-rebuild — it fired
before the force_rebuild handling, so a stale binary-store artifact could never
be regenerated. Guard it with !cook_config.force_rebuild so --force-rebuild always
takes effect (repo binary rebuilt).

kf6-kwayland: declare redbear-input-headers (fakeinput.cpp needs <linux/input.h>).

local/recipes/kde/kf6-kwayland/recipe.toml

@@ -16,6 +16,8 @@ dependencies = [
     "libwayland",
     "wayland-protocols",
     "plasma-wayland-protocols",
+    # <linux/input.h> for fakeinput.cpp (Linux input event codes).
+    "redbear-input-headers",
 ]
 script = """
 DYNAMIC_INIT

local/recipes/kde/plasma-wayland-protocols/recipe.toml

@@ -38,7 +38,7 @@ cat > "${COOKBOOK_STAGE}/usr/lib/cmake/PlasmaWaylandProtocols/PlasmaWaylandProto
 set(PLASMA_WAYLAND_PROTOCOLS_DIR "${CMAKE_CURRENT_LIST_DIR}/../../../share/plasma-wayland-protocols")
 EOF
 cat > "${COOKBOOK_STAGE}/usr/lib/cmake/PlasmaWaylandProtocols/PlasmaWaylandProtocolsConfigVersion.cmake" << 'EOF'
-set(PACKAGE_VERSION "1.16.0")
+set(PACKAGE_VERSION "1.21.0")
 if(PACKAGE_FIND_VERSION VERSION_GREATER PACKAGE_VERSION)
   set(PACKAGE_VERSION_COMPATIBLE FALSE)
 else()

src/cook/cook_build.rs

@@ -496,7 +496,12 @@ pub fn build(
         };
     }
 
-    if !check_source {
+    // Dependencies normally skip the source check for speed. But --force-rebuild
+    // must always take effect here — otherwise this early-return fires before the
+    // force_rebuild handling below, so a stale binary-store artifact whose source
+    // changed could never be regenerated (the KWayland/plasma-wayland-protocols
+    // 1.16-vs-1.21 stale-cache bug).
+    if !check_source && !cook_config.force_rebuild {
         // TODO: when stage_dirs does not exist due to clean_target was true, extract from stage.pkgar?
         let stage_present = stage_pkgars.iter().all(|file| file.is_file());
         if stage_present && auto_deps_file.is_file() {