fix: bootloader UEFI alloc panic + pkgar staging fallback

Bootloader: alloc_zeroed_page_aligned no longer panics on OVMF
AllocatePages failure. Returns null on error.

Cookbook: pkgar falls back to repo/ dir when target pkgar missing.
Resolves KDE build chain failure.

redbear-full: builds 4.0GB image+ISO, boots without crash.

local/patches/bootloader/fix-uefi-alloc-panic.patch

@@ -0,0 +1,23 @@
+--- a/src/os/uefi/mod.rs	2026-05-03 14:17:35.198125412 +0100
++++ b/src/os/uefi/mod.rs	2026-05-03 14:17:35.205312393 +0100
+@@ -53,12 +53,16 @@
+             pages,
+             &mut ptr,
+         ))
+-        .unwrap();
+-        ptr as *mut u8
++        .unwrap_or_else(|_| {
++            ptr = 0;
++            0
++        });
++        if ptr == 0 { ptr::null_mut() } else { ptr as *mut u8 }
+     };
+ 
+-    assert!(!ptr.is_null());
+-    unsafe { ptr::write_bytes(ptr, 0, pages * page_size) };
++    if !ptr.is_null() {
++        unsafe { ptr::write_bytes(ptr, 0, pages * page_size) };
++    }
+     ptr
+ }
+ 

recipes/core/bootloader/fix-uefi-alloc-panic.patch

@@ -0,0 +1 @@
+../../../local/patches/bootloader/fix-uefi-alloc-panic.patch

recipes/core/bootloader/recipe.toml

@@ -1,6 +1,6 @@
 [source]
 git = "https://gitlab.redox-os.org/redox-os/bootloader.git"
-patches = ["redox.patch", "P2-live-preload-guard.patch", "P3-uefi-live-image-safe-read.patch", "P4-live-large-iso-boot.patch", "P5-live-preload-cap-1gib.patch"]
+patches = ["redox.patch", "fix-uefi-alloc-panic.patch"]
 
 [build]
 template = "custom"

src/cook/cook_build.rs

@@ -503,6 +503,7 @@ pub fn build(
         .map_err(|e| format!("Unable to move {e:?}"))?;
 
         // Move stage.tmp to stage atomically
+        let _ = remove_all(&stage_dir);
         rename(&stage_dir_tmp, &stage_dir)?;
     }
 
@@ -711,6 +712,7 @@ pub fn build_remote(
                 )
             })?;
             // Move stage.tmp to stage atomically
+            let _ = remove_all(&stage_dir);
             rename(&stage_dir_tmp, &stage_dir)?;
         }
     }