fix(interpreter): expand parameter operators inside arithmetic base# expressions (#1039)

## Summary
- Add `expand_param_op_in_arithmetic` to handle `${var%%-*}`,
`${var##prefix}`, etc. inside arithmetic
- Detect parameter operators in `expand_brace_expr_in_arithmetic` and
delegate to new handler
- Fixes `$(( 10#${var%%-*} ))` returning 0 instead of the correct
base-10 value

## Test plan
- [x] Spec tests: 4 cases (suffix removal, prefix removal, arithmetic,
plain var)
- [x] Full test suite passes

Closes #944

Author: chaliy

crates/bashkit/src/interpreter/mod.rs

@@ -7610,10 +7610,67 @@ impl Interpreter {
             return String::new();
         }
 
+        // Check for parameter expansion operators (%, %%, #, ##, :-, etc.)
+        // If present, handle expansion with the operator applied.
+        let has_operator = inner.contains("%%")
+            || inner.contains('%')
+            || (inner.contains('#') && !inner.starts_with('#'))
+            || inner.contains(":-");
+        if has_operator {
+            return self.expand_param_op_in_arithmetic(inner);
+        }
+
         // ${var} — plain variable
         self.expand_variable(inner)
     }
 
+    /// Expand a parameter expansion with operators inside arithmetic context.
+    /// Handles common cases like ${var%%-*}, ${var##prefix}, etc.
+    fn expand_param_op_in_arithmetic(&self, inner: &str) -> String {
+        // ${var%%pattern} — remove longest suffix
+        if let Some(pos) = inner.find("%%") {
+            let name = &inner[..pos];
+            let pattern = &inner[pos + 2..];
+            let value = self.expand_variable(name);
+            return self.remove_pattern(&value, pattern, false, true);
+        }
+        // ${var%pattern} — remove shortest suffix
+        if let Some(pos) = inner.find('%') {
+            let name = &inner[..pos];
+            let pattern = &inner[pos + 1..];
+            let value = self.expand_variable(name);
+            return self.remove_pattern(&value, pattern, false, false);
+        }
+        // ${var##pattern} — remove longest prefix
+        if let Some(pos) = inner.find("##") {
+            let name = &inner[..pos];
+            let pattern = &inner[pos + 2..];
+            let value = self.expand_variable(name);
+            return self.remove_pattern(&value, pattern, true, true);
+        }
+        // ${var#pattern} — remove shortest prefix (but not ${#var} length)
+        if let Some(pos) = inner.find('#')
+            && pos > 0
+        {
+            let name = &inner[..pos];
+            let pattern = &inner[pos + 1..];
+            let value = self.expand_variable(name);
+            return self.remove_pattern(&value, pattern, true, false);
+        }
+        // ${var:-default}
+        if let Some(pos) = inner.find(":-") {
+            let name = &inner[..pos];
+            let default = &inner[pos + 2..];
+            let value = self.expand_variable(name);
+            if value.is_empty() {
+                return default.to_string();
+            }
+            return value;
+        }
+        // Fallback
+        self.expand_variable(inner)
+    }
+
     /// Parse and evaluate a simple arithmetic expression with depth tracking.
     /// THREAT[TM-DOS-026]: `arith_depth` prevents stack overflow from deeply nested expressions.
     /// Parse an arithmetic atom: unary operators, parenthesized expressions, and literals.

crates/bashkit/tests/spec_cases/bash/arithmetic-base-expansion.test.sh

@@ -0,0 +1,31 @@
+### arithmetic_base_with_suffix_removal
+# 10#${var%%-*} should expand then convert
+last="0003-assistant.md"
+echo $(( 10#${last%%-*} ))
+### expect
+3
+### end
+
+### arithmetic_base_with_prefix_removal
+# 10#${var##0} should expand then convert
+val="007"
+echo $(( 10#${val##0} ))
+### expect
+7
+### end
+
+### arithmetic_base_with_expansion_plus
+# 10#${var%%-*} + 1 in arithmetic
+seq="0041-user.md"
+echo $(( 10#${seq%%-*} + 1 ))
+### expect
+42
+### end
+
+### arithmetic_base_simple_var
+# 10#${var} without operators (verify no regression)
+x="0099"
+echo $(( 10#${x} ))
+### expect
+99
+### end