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/riscv64.rs

@@ -0,0 +1,47 @@
+use core::mem;
+use syscall::{data::Map, flag::MapFlags, number::SYS_FMAP};
+
+const STACK_SIZE: usize = 64 * 1024; // 64 KiB
+pub const USERMODE_END: usize = 1 << 38; // Assuming Sv39
+pub const STACK_START: usize = USERMODE_END - syscall::KERNEL_METADATA_SIZE - STACK_SIZE;
+
+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.
+    li a7, {number}
+    li a0, {fd}
+    la a1, {map} # pointer to Map struct
+    li a2, {map_size} # size of Map struct
+    ecall
+
+    # Test for success (nonzero value).
+    bne a0, x0, 2f
+    # (failure)
+    unimp
+2:
+    li sp, {stack_size}
+    add sp, sp, a0
+    mv fp, x0
+
+    jal start
+    # `start` must never return.
+    unimp
+    ",
+    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,
+);