fix(interpreter): expand command substitutions in assoc array keys

Associative array assignments where the key is a command substitution
(e.g. m["$(echo hello)"]="world") silently produced an empty key.

Add async expand_assoc_key() that parses the subscript as a full Word
and expands it with expand_word() when it contains $( or backtick.

Closes #872

Author: chaliy

crates/bashkit/src/interpreter/mod.rs

@@ -3129,7 +3129,7 @@ impl Interpreter {
                     if let Some(index_str) = &assignment.index {
                         let resolved_name = self.resolve_nameref(&assignment.name).to_string();
                         if self.assoc_arrays.contains_key(&resolved_name) {
-                            let key = self.expand_variable_or_literal(index_str);
+                            let key = self.expand_assoc_key(index_str).await?;
                             let is_new_entry = self
                                 .assoc_arrays
                                 .get(&resolved_name)
@@ -7791,6 +7791,21 @@ impl Interpreter {
         s.to_string()
     }
 
+    /// Fully expand an associative array key, including command substitutions.
+    /// Falls back to `expand_variable_or_literal` for keys without `$(` or backtick.
+    async fn expand_assoc_key(&mut self, s: &str) -> Result<String> {
+        if s.contains("$(") || s.contains('`') {
+            let word = Parser::parse_word_string_with_limits(
+                s,
+                self.limits.max_ast_depth,
+                self.limits.max_parser_operations,
+            );
+            self.expand_word(&word).await
+        } else {
+            Ok(self.expand_variable_or_literal(s))
+        }
+    }
+
     /// THREAT[TM-INJ-009]: Check if a variable name is an internal marker.
     fn is_internal_variable(name: &str) -> bool {
         is_internal_variable(name)

crates/bashkit/tests/spec_cases/bash/assoc-arrays.test.sh

@@ -173,6 +173,16 @@ b=2
 c=3
 ### end
 
+### assoc_key_command_substitution
+declare -A m=()
+m["$(echo hello)"]="world"
+echo "count: ${#m[@]}"
+for k in "${!m[@]}"; do echo "key=[$k] val=[${m[$k]}]"; done
+### expect
+count: 1
+key=[hello] val=[world]
+### end
+
 ### assoc_iteration
 declare -A m
 m[a]="1"