feat(fuzz): add template_fuzz target (#1147)

## Summary
- Add fuzz target for the custom Mustache/Handlebars template engine in
`builtins/template.rs`
- Tests template rendering with JSON data input, `--strict` mode, and
`-e` HTML escaping
- Guards against deeply nested `{{#...}}` blocks

Closes #1098

## Test plan
- [x] `cargo check` in fuzz crate passes
- [x] `cargo fmt --check` clean
- [x] `cargo clippy` clean

Author: chaliy

crates/bashkit/fuzz/Cargo.toml

@@ -101,3 +101,10 @@ path = "fuzz_targets/yaml_fuzz.rs"
 test = false
 doc = false
 bench = false
+
+[[bin]]
+name = "template_fuzz"
+path = "fuzz_targets/template_fuzz.rs"
+test = false
+doc = false
+bench = false

crates/bashkit/fuzz/fuzz_targets/template_fuzz.rs

@@ -0,0 +1,82 @@
+//! Fuzz target for the template builtin
+//!
+//! Tests the custom Mustache/Handlebars template engine to find:
+//! - Panics on mismatched delimiters or deeply nested sections
+//! - Stack overflow from recursive block expansion
+//! - Edge cases in variable substitution and control flow blocks
+//! - Memory exhaustion from pathological template patterns
+//!
+//! Run with: cargo +nightly fuzz run template_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;
+        }
+
+        // Split input into template (first line) and JSON data (rest)
+        let (template, json_data) = match input.find('\n') {
+            Some(pos) => (&input[..pos], &input[pos + 1..]),
+            None => (input, "{\"name\":\"world\",\"items\":[1,2,3]}" as &str),
+        };
+
+        // Skip empty templates
+        if template.trim().is_empty() {
+            return;
+        }
+
+        // Reject deeply nested template blocks
+        let depth = template.matches("{{#").count();
+        if depth > 10 {
+            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(4096)
+                        .max_stderr_bytes(4096)
+                        .timeout(std::time::Duration::from_millis(200)),
+                )
+                .build();
+
+            // Test 1: render template with JSON data via stdin
+            let script = format!(
+                "echo '{}' | template -d /dev/stdin '{}' 2>/dev/null; true",
+                json_data.replace('\'', "'\\''"),
+                template.replace('\'', "'\\''"),
+            );
+            let _ = bash.exec(&script).await;
+
+            // Test 2: render template with --strict mode
+            let script2 = format!(
+                "echo '{}' | template --strict -d /dev/stdin '{}' 2>/dev/null; true",
+                json_data.replace('\'', "'\\''"),
+                template.replace('\'', "'\\''"),
+            );
+            let _ = bash.exec(&script2).await;
+
+            // Test 3: render template with -e (HTML escape) flag
+            let script3 = format!(
+                "echo '{}' | template -e -d /dev/stdin '{}' 2>/dev/null; true",
+                json_data.replace('\'', "'\\''"),
+                template.replace('\'', "'\\''"),
+            );
+            let _ = bash.exec(&script3).await;
+        });
+    }
+});