feat(fuzz): add base64_fuzz target (#1143)

## Summary
- Add fuzz target for the `base64` builtin covering encode/decode with
arbitrary data
- Tests invalid base64 sequences, wrong padding, truncated input,
roundtrips, and wrap width options

## Test plan
- [x] `cargo check` passes in fuzz directory
- [ ] CI green

Closes #1104

Author: chaliy

crates/bashkit/fuzz/Cargo.toml

@@ -73,3 +73,10 @@ path = "fuzz_targets/printf_fuzz.rs"
 test = false
 doc = false
 bench = false
+
+[[bin]]
+name = "base64_fuzz"
+path = "fuzz_targets/base64_fuzz.rs"
+test = false
+doc = false
+bench = false

crates/bashkit/fuzz/fuzz_targets/base64_fuzz.rs

@@ -0,0 +1,69 @@
+//! Fuzz target for the base64 builtin
+//!
+//! Tests base64 encode/decode to find:
+//! - Panics on invalid base64 sequences or wrong padding
+//! - Encode/decode roundtrip mismatches
+//! - Truncated input handling
+//! - Edge cases with wrap width and -d flag
+//!
+//! Run with: cargo +nightly fuzz run base64_fuzz -- -max_total_time=300
+
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| {
+    // Only process valid UTF-8
+    if let Ok(input) = std::str::from_utf8(data) {
+        // Limit input size to prevent OOM
+        if input.len() > 1024 {
+            return;
+        }
+
+        let rt = tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap();
+
+        rt.block_on(async {
+            let mut bash = bashkit::Bash::builder()
+                .limits(
+                    bashkit::ExecutionLimits::new()
+                        .max_commands(50)
+                        .max_subst_depth(3)
+                        .max_stdout_bytes(8192)
+                        .max_stderr_bytes(4096)
+                        .timeout(std::time::Duration::from_millis(200)),
+                )
+                .build();
+
+            // Test 1: encode arbitrary data
+            let script = format!(
+                "echo -n '{}' | base64 2>/dev/null; true",
+                input.replace('\'', "'\\''"),
+            );
+            let _ = bash.exec(&script).await;
+
+            // Test 2: decode arbitrary data (may be invalid base64)
+            let script2 = format!(
+                "echo -n '{}' | base64 -d 2>/dev/null; true",
+                input.replace('\'', "'\\''"),
+            );
+            let _ = bash.exec(&script2).await;
+
+            // Test 3: encode then decode roundtrip
+            let script3 = format!(
+                "echo -n '{}' | base64 | base64 -d 2>/dev/null; true",
+                input.replace('\'', "'\\''"),
+            );
+            let _ = bash.exec(&script3).await;
+
+            // Test 4: decode with --wrap=0
+            let script4 = format!(
+                "echo -n '{}' | base64 --wrap=0 2>/dev/null; true",
+                input.replace('\'', "'\\''"),
+            );
+            let _ = bash.exec(&script4).await;
+        });
+    }
+});