fix(rumpk): Fix LwIP kernel build for RISC-V freestanding

- Rebuild liblwip.a from clean sources (removed initrd.o contamination)
- Add switch.o to provide cpu_switch_to symbol
- Add sys_arch.o to provide sys_now and nexus_lwip_panic
- Add freestanding defines to cc.h (LWIP_NO_CTYPE_H, etc.)
- Compile sys_arch.c with -mcmodel=medany for RISC-V

Fixes duplicate symbol errors and undefined reference errors.
Kernel now builds successfully with: zig build -Dtarget=riscv64-freestanding

build.zig

@@ -29,6 +29,7 @@ pub fn build(b: *std.Build) void {
     // Freestanding kernel - no libc, no red zone, no stack checks
     hal_mod.red_zone = false;
     hal_mod.stack_check = false;
+    hal_mod.code_model = .medany;
 
     const hal = b.addLibrary(.{
         .name = "rumpk_hal",
@@ -58,13 +59,60 @@ pub fn build(b: *std.Build) void {
     });
     boot_mod.red_zone = false;
     boot_mod.stack_check = false;
+    boot_mod.code_model = .medany;
 
     const boot = b.addObject(.{
         .name = "boot",
         .root_module = boot_mod,
     });
 
-    _ = boot; // Mark as used for now
+    // =========================================================
+    // Final Link: rumpk.elf
+    // =========================================================
+
+    const kernel_mod = b.createModule(.{
+        .root_source_file = b.path("hal/abi.zig"), // Fake root, we add objects later
+        .target = target,
+        .optimize = optimize,
+    });
+    kernel_mod.red_zone = false;
+    kernel_mod.stack_check = false;
+    kernel_mod.code_model = .medany;
+
+    const kernel = b.addExecutable(.{
+        .name = "rumpk.elf",
+        .root_module = kernel_mod,
+    });
+
+    kernel.setLinkerScript(b.path("boot/linker.ld"));
+    kernel.addObject(boot);
+    // kernel.linkLibrary(hal); // Redundant, already in kernel_mod
+
+    // Add Nim-generated objects
+    {
+        var nimcache_dir = std.fs.cwd().openDir("build/nimcache", .{ .iterate = true }) catch |err| {
+            std.debug.print("Warning: Could not open nimcache dir: {}\n", .{err});
+            return;
+        };
+        defer nimcache_dir.close();
+        var it = nimcache_dir.iterate();
+        while (it.next() catch null) |entry| {
+            if (entry.kind == .file and std.mem.endsWith(u8, entry.name, ".o")) {
+                const path = b.fmt("build/nimcache/{s}", .{entry.name});
+                kernel.addObjectFile(b.path(path));
+            }
+        }
+    }
+
+    // Add external pre-built dependencies (Order matters: Libs after users)
+    kernel.addObjectFile(b.path("build/switch.o")); // cpu_switch_to
+    kernel.addObjectFile(b.path("build/sys_arch.o")); // sys_now, nexus_lwip_panic
+    kernel.addObjectFile(b.path("build/libc_shim.o"));
+    kernel.addObjectFile(b.path("build/clib.o"));
+    kernel.addObjectFile(b.path("build/liblwip.a"));
+    kernel.addObjectFile(b.path("build/initrd.o"));
+
+    b.installArtifact(kernel);
 
     // =========================================================
     // Tests

libs/membrane/include/arch/cc.h

@@ -16,9 +16,18 @@
 #ifndef LWIP_ARCH_CC_H
 #define LWIP_ARCH_CC_H
 
+// =========================================================
+// Freestanding Environment - Disable unavailable headers
+// =========================================================
+#define LWIP_NO_CTYPE_H     1  // ctype.h not available
+#define LWIP_NO_LIMITS_H    1  // limits.h not available
+#define LWIP_NO_UNISTD_H    1  // unistd.h not available
+#define LWIP_NO_INTTYPES_H  1  // inttypes.h not available
+
 #include <stdint.h>
 #include <stddef.h>
 
+
 // =========================================================
 // Basic Types (Fixed-width integers)
 // =========================================================