milestone: desktop path Phases 1-5

Phase 1 (Runtime Substrate): 4 check binaries, --probe, POSIX tests
Phase 2 (Wayland Compositor): bounded scaffold, zero warnings
Phase 3 (KWin Session): preflight checker (KWin stub, gated on Qt6Quick)
Phase 4 (KDE Plasma): 18 KF6 enabled, preflight checker
Phase 5 (Hardware GPU): DRM/firmware/Mesa preflight checker

Build: zero warnings, all scripts syntax-clean. Oracle-verified.

recipes/core/base/bootstrap/src/aarch64.rs

@@ -0,0 +1,53 @@
+use core::mem;
+use syscall::{data::Map, flag::MapFlags, number::SYS_FMAP};
+
+pub const USERMODE_END: usize = 0x0000_8000_0000_0000;
+pub const STACK_START: usize = USERMODE_END - syscall::KERNEL_METADATA_SIZE - STACK_SIZE;
+
+const STACK_SIZE: usize = 64 * 1024; // 64 KiB
+static MAP: Map = Map {
+    offset: 0,
+    size: STACK_SIZE,
+    flags: MapFlags::PROT_READ
+        .union(MapFlags::PROT_WRITE)
+        .union(MapFlags::MAP_PRIVATE)
+        .union(MapFlags::MAP_FIXED_NOREPLACE),
+    address: STACK_START, // highest possible user address
+};
+
+core::arch::global_asm!(
+    "
+    .globl _start
+    _start:
+    // Setup a stack.
+    ldr x8, ={number}
+    ldr x0, ={fd}
+    ldr x1, ={map} // pointer to Map struct
+    ldr x2, ={map_size} // size of Map struct
+    svc 0
+
+    // Failure if return value is zero
+    cbz x0, 1f
+
+    // Failure if return value is negative
+    tbnz x0, 63, 1f
+
+    // Set up stack frame
+    mov sp, x0
+    add sp, sp, #{stack_size}
+    mov fp, sp
+
+    // Stack has the same alignment as `size`.
+    bl start
+    // `start` must never return.
+
+    // failure, emit undefined instruction
+    1:
+    udf #0
+    ",
+    fd = const usize::MAX, // dummy fd indicates anonymous map
+    map = sym MAP,
+    map_size = const mem::size_of::<Map>(),
+    number = const SYS_FMAP,
+    stack_size = const STACK_SIZE,
+);