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/types/src/bounded.rs

@@ -3,3 +3,8 @@ use crate::macros::bounded;
 bounded!(CapSlot, u8, 0, 63);
 bounded!(Priority, u8, 0, 31);
 bounded!(PageCount, u16, 1, 1024);
+
+impl Priority {
+    pub const DEFAULT: Self = Self(16);
+}
+

crates/types/src/time.rs

@@ -25,6 +25,11 @@ pub struct TimeSlice {
 }
 
 impl TimeSlice {
+    pub const DEFAULT: Self = Self {
+        remaining: 100,
+        quantum: 100,
+    };
+
     pub fn new(quantum: u32) -> Self {
         Self {
             remaining: quantum,