Add a string_repeat builtin for repeating a string N times.

Proposed function

• string_repeat(@String, @Nat -> @String) -- repeat string N times

Rationale

Building repeated patterns (padding, separators, indentation) currently requires recursive functions with manual string_concat calls. A dedicated builtin is simpler and more efficient.

Implementation notes

Allocate length * N bytes, copy the source string N times in a loop. Pure function.

Dependencies

• None