execute_simple_command stack: 171KB -> 13KB, enabling ~400 level
recursion depth (up from 33) with default 8MB stack.

- temp_tokens uses string_t for variable-length per-element storage
- split_words dynamically resized when expanded values exceed initial capacity
- process_command_escapes uses stack-scoped result buffer sized to token array width
- copy-back computes max token length and re-allocates cmd%tokens to fit

is_valid_assignment_name and is_valid_var_name now stop validation at '[' so
arr[0]=$x is classified as an assignment instead of a command word. Fixes
variable expansion and command substitution in indexed/associative array
element assignments (STRESS 3.2, 3.3, 4.2, 4.3).