refactor(adapter-riscv): restructure into hw/, mmu/, trap/ modules with board constants

Add Priority::DEFAULT and TimeSlice::DEFAULT to types crate.
All QEMU virt addresses consolidated in board.rs.
Structure mirrors future adapter-arm/ layout.

crates/adapter-riscv/src/mmu/allocator.rs

@@ -0,0 +1,19 @@
+use super::page_table::{PageTable, PAGE_SIZE};
+
+// Boot-time bump allocator for page tables.
+// Only used during init before the scheduler starts.
+static mut NEXT_FREE: usize = 0;
+
+pub fn init_allocator(start: usize) {
+    unsafe { NEXT_FREE = start; }
+}
+
+pub fn alloc_page_table() -> &'static mut PageTable {
+    unsafe {
+        let addr = NEXT_FREE;
+        NEXT_FREE += PAGE_SIZE;
+        let table = &mut *(addr as *mut PageTable);
+        table.zero();
+        table
+    }
+}