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/userspace-rt/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");
+}

crates/userspace-rt/linker.ld

@@ -0,0 +1,25 @@
+OUTPUT_ARCH(riscv)
+ENTRY(_start)
+
+SECTIONS {
+    . = 0x10000;
+
+    .text : {
+        *(.text.entry)
+        *(.text .text.*)
+    }
+
+    . = ALIGN(4096);
+    .rodata : { *(.rodata .rodata.*) }
+
+    . = ALIGN(4096);
+    .data : { *(.data .data.*) }
+
+    . = ALIGN(4096);
+    .bss : {
+        *(.bss .bss.*)
+        *(.sbss .sbss.*)
+    }
+
+    /DISCARD/ : { *(.eh_frame) }
+}