refactor(adapter-riscv): split kmain into boot/, devices/ modules

kmain is now 20 lines — six function calls:

  hardware::init        → BSS, MMU, traps, timer, PLIC, kernel
  loader::load_init     → read init.elf, create process, map ELF
  loader::load_server   → read server ELF, create process, map ELF
  storage_setup         → VirtIO block re-init, shared buffer, phys-info
  gpu_setup             → VirtIO GPU init, framebuffer alloc, phys-info
  caps::grant_all       → endpoints, capability grants

New modules:
  boot/hardware.rs   — hardware init (BSS, MMU, traps, kernel)
  boot/loader.rs     — ELF loading, thread setup, enter_userspace
  boot/caps.rs       — endpoint creation, capability grants
  devices/storage_setup.rs — VirtIO block for storage server
  devices/gpu_setup.rs     — VirtIO GPU for display server

All unsafe contained in hw/, mmu/, trap/. Boot and device
modules call safe wrappers.

crates/adapter-riscv/src/boot/hardware.rs

@@ -0,0 +1,52 @@
+use crate::hw::uart::Uart;
+use crate::hw::virtio_blk::VirtioBlk;
+use crate::{board, kernel_cell::KernelCell, mmu, platform, trap, KERNEL};
+
+unsafe extern "C" {
+    static _bss_start: u8;
+    static _bss_end: u8;
+    static _heap_start: u8;
+}
+
+pub fn init(uart: &Uart) -> &'static mut mmu::PageTable {
+    zero_bss();
+
+    let heap_start = unsafe { &_heap_start as *const u8 as usize };
+    mmu::init_allocator(heap_start);
+    let kernel_pt = mmu::init_kernel_page_table();
+    mmu::activate(kernel_pt);
+    uart.write_str("k-os: MMU enabled (Sv39)\n");
+
+    trap::init();
+    crate::hw::timer::init();
+    crate::hw::plic::Plic::init();
+    uart.write_str("k-os: traps and timer initialized\n");
+
+    let riscv_platform = platform::RiscVPlatform::new(kernel_pt as *const mmu::PageTable as usize);
+    let pool = domain::memory::page_pool::PagePool::new(
+        types::address::PhysicalAddress::from_raw(board::USER_POOL_BASE),
+        board::USER_POOL_PAGES,
+    );
+    KERNEL.init(kernel::Kernel::new(riscv_platform, pool));
+
+    kernel_pt
+}
+
+pub fn probe_block_device(uart: &Uart) -> Option<VirtioBlk> {
+    let blk = VirtioBlk::probe();
+    if blk.is_some() {
+        uart.write_str("k-os: VirtIO block device found\n");
+    } else {
+        uart.write_str("k-os: no block device\n");
+    }
+    blk
+}
+
+fn zero_bss() {
+    unsafe {
+        let start = &_bss_start as *const u8 as *mut u8;
+        let end = &_bss_end as *const u8 as *mut u8;
+        let len = end as usize - start as usize;
+        core::ptr::write_bytes(start, 0, len);
+    }
+}