fix(interpreter): initialize assoc/indexed arrays for local -A/-a

[chaliy]

Summary

• local -A m followed by m["a"]="1" produced an indexed array instead of associative, because execute_local_builtin only inserted into call_stack.locals without creating the assoc_arrays entry
• Initialize the appropriate array type when local -A or -a is used without a compound assignment value
• Fix applies to both function-scope and global-scope paths
• Add spec tests assoc_local_declare_then_assign and assoc_local_keys_in_cmdsub

Test plan

• New spec tests pass
• All 1812 bash spec tests pass (100%)
• Full cargo test --all-features passes
• cargo fmt --check and cargo clippy clean

Closes #875