[P2] String helpers: capitalize, pad-left/pad-right, repeat

[ecto]

The Loon string builtins cover the basics (uppercase, lowercase,
trim, split, join, replace, starts-with?, ends-with?,
contains?, index-of, substring, char-at, len), but a few
common operations are missing:

• capitalize — uppercase first character, lowercase the rest
• pad-left / pad-right — pad a string to a target width
• repeat — repeat a string N times (e.g. [repeat "-" 10])

The modules guide shows capitalize as an example user function,
suggesting demand.

Acceptance criteria

• Builtins available in the VM and documented in ref/builtins.loon.
• Tests for empty strings, unicode strings, and width-shorter-than-input
  for the pad functions.