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/build.rs

@@ -0,0 +1,5 @@
+fn main() {
+    let ld = std::path::Path::new("linker.ld").canonicalize().unwrap();
+    println!("cargo:rustc-link-arg=-T{}", ld.display());
+    println!("cargo:rerun-if-changed=linker.ld");
+}