feat(rust,gui): RLE compression codec + Rust Lab desktop app

Rust kernel expansion:
- rust/src/compress/mod.rs (new): memory-safe PackBits-style run-length
  encoding codec (rle_compress / rle_decompress) with C-ABI shims, registered
  in lib.rs. C fallbacks added to rust_fallback.c so aarch64/armv8i match.
- include/kernel/rust.h: FFI declarations for the codec.
- LazyBox: new `rle <text>` applet that compresses, decompresses, prints the
  ratio and verifies the round-trip is lossless.

GUI expansion:
- gui/gui_apps.c: "Rust Lab" app -- a live dashboard for the Rust subsystem.
  Shows CRC-32C/Adler-32 of a sample, measured ChaCha20/SHA3-256/AES-128
  throughput as scaled bars plus a composite score (benchmark run once at
  launch and cached), and a live ChaCha20 CSPRNG byte stream refreshed
  periodically. Per-window state freed on close.
- Registered "Rust Lab" (info icon) in the desktop app table and on the
  desktop; desktop icon indices updated for the new entry.

Verified: clean build on x86_64/aarch64/armv8i; boots through the SUB login to
the desktop with the new Rust Lab icon rendering (screenshotted); checksum
self-test prints, 0 panics / 0 OOM.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Fh5z6V7nGPbHvqVG5p4CXX

rust/src/compress/mod.rs

@@ -0,0 +1,93 @@
+//! Memory-safe run-length encoding (RLE) codec for SUB-OS.
+//!
+//! A compact PackBits-style byte codec: the stream is a sequence of
+//! (count, value) pairs where `count` is 1..=255. It is intended for run-heavy
+//! data (framebuffer scanlines, zeroed pages, simple bitmaps) and is fully
+//! `no_std` with no allocation -- the caller owns both buffers.
+
+/// Compress `input` into `output` as (count, value) pairs. Returns the number
+/// of bytes written, or -1 if `output` is too small.
+pub fn rle_compress(input: &[u8], output: &mut [u8]) -> isize {
+    let mut out = 0usize;
+    let mut i = 0usize;
+    let len = input.len();
+
+    while i < len {
+        let value = input[i];
+        let mut run = 1usize;
+        while i + run < len && input[i + run] == value && run < 255 {
+            run += 1;
+        }
+        if out + 2 > output.len() {
+            return -1;
+        }
+        output[out] = run as u8;
+        output[out + 1] = value;
+        out += 2;
+        i += run;
+    }
+    out as isize
+}
+
+/// Expand a (count, value) stream back into `output`. Returns the number of
+/// bytes written, or -1 on a malformed stream or an output overflow.
+pub fn rle_decompress(input: &[u8], output: &mut [u8]) -> isize {
+    let mut out = 0usize;
+    let mut i = 0usize;
+    let len = input.len();
+
+    while i + 1 < len {
+        let run = input[i] as usize;
+        let value = input[i + 1];
+        if run == 0 {
+            return -1; // a zero run is never emitted by the compressor
+        }
+        if out + run > output.len() {
+            return -1;
+        }
+        for _ in 0..run {
+            output[out] = value;
+            out += 1;
+        }
+        i += 2;
+    }
+    out as isize
+}
+
+// ---------------------------------------------------------------------------
+// C ABI shims
+// ---------------------------------------------------------------------------
+
+#[no_mangle]
+pub extern "C" fn rust_rle_compress(
+    in_buf: *const u8,
+    in_len: usize,
+    out_buf: *mut u8,
+    max_out: usize,
+) -> i32 {
+    if in_buf.is_null() || out_buf.is_null() {
+        return -1;
+    }
+    unsafe {
+        let input = core::slice::from_raw_parts(in_buf, in_len);
+        let output = core::slice::from_raw_parts_mut(out_buf, max_out);
+        rle_compress(input, output) as i32
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn rust_rle_decompress(
+    in_buf: *const u8,
+    in_len: usize,
+    out_buf: *mut u8,
+    max_out: usize,
+) -> i32 {
+    if in_buf.is_null() || out_buf.is_null() {
+        return -1;
+    }
+    unsafe {
+        let input = core::slice::from_raw_parts(in_buf, in_len);
+        let output = core::slice::from_raw_parts_mut(out_buf, max_out);
+        rle_decompress(input, output) as i32
+    }
+}