feat: userspace linker script at 0x10000, proper ELF segment loading, full disk boot pipeline working

- Userspace linker script: apps/servers link at 0x10000 (user VA)
- Adapter linker script: kernel links at 0x80200000 (S-mode)
- Each uses build.rs to emit its own -T flag, global config only sets panic=abort
- ELF loader: page-aligns segment mappings, handles non-aligned vaddrs
- Stack: 4 pages (16K) mapped below program
- SP set to top of stack (0x10000), not bottom
- FAT16 disk image forced (mformat was creating FAT32)
- End-to-end verified: VirtIO → FAT16 → ELF → userspace → ecall → exit

crates/adapter-riscv/src/platform.rs

@@ -30,6 +30,10 @@ impl RiscVPlatform {
         self.page_tables[pid.raw() as usize]
     }
 
+    pub fn set_page_table(&mut self, pid: ProcessId, addr: usize) {
+        self.page_tables[pid.raw() as usize] = addr;
+    }
+
     fn ensure_page_table(&mut self, pid: ProcessId) -> &mut mmu::PageTable {
         let idx = pid.raw() as usize;
         if self.page_tables[idx] == 0 {