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/hw/mod.rs

@@ -0,0 +1,3 @@
+pub mod plic;
+pub mod timer;
+pub mod uart;