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

.cargo/config.toml

@@ -1,3 +1,3 @@
 [target.riscv64gc-unknown-none-elf]
 runner = "qemu-system-riscv64 -machine virt -nographic -bios default -kernel"
-rustflags = ["-C", "link-arg=-Tcrates/adapter-riscv/linker.ld", "-C", "panic=abort"]
+rustflags = ["-C", "panic=abort"]