Refactor and enhance filesystem navigation and communication between Katas

- Updated `mi.zig` to default target path to an empty string.
- Improved context switching in `context.zig` by restoring registers from the context struct.
- Enhanced `loader.zig` to inherit current location from the parent Kata.
- Simplified `getkeychar.zig` by removing unnecessary serial import and error handling.
- Added new invocations: `getlocation.zig` for retrieving current stack location and `postman.zig` for letter passing between Katas.
- Introduced `setlocation.zig` to change current stack location with path resolution.
- Created `nav.zig` for navigating the filesystem with commands to print current location or change it.
- Added `nav.zon` for binary metadata.
- Updated `limits.zig` to define maximum letter length.
- Enhanced `ash.zig` to process navigation letters and improved command execution flow.
- Updated syscall interface in `sys.zig` to handle variable arguments for invocations.

3 files changed, 58 insertions, 2 deletions

diff --git a/binaries/mi/mi.zig b/binaries/mi/mi.zig
index 0f5e400..1e158a9 100644
--- a/binaries/mi/mi.zig
+++ b/binaries/mi/mi.zig
@@ -18,8 +18,8 @@ const PERM_WORLD: u8 = 2;
 const PERM_READ_ONLY: u8 = 3;
 
 export fn main(pc: u32, pv: [*]const [*:0]const u8) u8 {
-    // Use first argument as path, or default to "/"
-    var target_path: []const u8 = "/";
+    // Use first argument as path, or default to ""
+    var target_path: []const u8 = "";
 
     if (pc > 1) {
         // Get pv[1] as the target path
diff --git a/binaries/nav/nav.zig b/binaries/nav/nav.zig
new file mode 100644
index 0000000..98883d0
--- /dev/null
+++ b/binaries/nav/nav.zig
@@ -0,0 +1,46 @@
+//! nav - Navigate the filesystem
+//!
+//! nav          - Print current location
+//! nav <path>   - Navigate to path
+//! nav ^        - Navigate to parent
+//! nav /        - Navigate to root
+
+const akiba = @import("akiba");
+
+const Color = struct {
+    const white: u32 = 0x00FFFFFF;
+    const green: u32 = 0x0088FF88;
+    const red: u32 = 0x00FF4444;
+};
+
+// Global buffer
+var location_buf: [256]u8 = undefined;
+
+export fn main(pc: u32, pv: [*]const [*:0]const u8) u8 {
+    // No arguments - print current location
+    if (pc <= 1) {
+        const location = akiba.io.getlocation(&location_buf) catch {
+            _ = akiba.io.mark(akiba.io.stream, "nav: cannot get current location.\n", Color.red) catch 0;
+            return 1;
+        };
+        _ = akiba.io.mark(akiba.io.stream, location, Color.green) catch 0;
+        _ = akiba.io.mark(akiba.io.stream, "\n", Color.white) catch 0;
+        return 0;
+    }
+
+    // Get target path from argument
+    const arg = pv[1];
+    var target_len: usize = 0;
+    while (arg[target_len] != 0) : (target_len += 1) {}
+    const target = arg[0..target_len];
+
+    // Navigate - setlocation auto-sends letter to parent
+    akiba.io.setlocation(target) catch {
+        _ = akiba.io.mark(akiba.io.stream, "nav: cannot navigate to '", Color.red) catch 0;
+        _ = akiba.io.mark(akiba.io.stream, target, Color.white) catch 0;
+        _ = akiba.io.mark(akiba.io.stream, "': No such stack.\n", Color.red) catch 0;
+        return 1;
+    };
+
+    return 0;
+}
diff --git a/binaries/nav/nav.zon b/binaries/nav/nav.zon
new file mode 100644
index 0000000..8597b44
--- /dev/null
+++ b/binaries/nav/nav.zon
@@ -0,0 +1,10 @@
+.{
+    .name = "nav",
+    .version = "0.1.0",
+    .dependencies = .{
+        .akiba = .{ .path = "../../system/libraries/akiba" },
+    },
+    .paths = .{
+        "nav.zig",
+    },
+}