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/mod.rs

@@ -0,0 +1,5 @@
+pub mod allocator;
+pub mod page_table;
+
+pub use allocator::init_allocator;
+pub use page_table::{activate, init_kernel_page_table, create_user_page_table, PageTable, PAGE_SIZE};