Unify escape/date helper API; fix current_date_text caching

Make the template.source.sh escape/date helper family follow one
consistent shape: each escaper now has a nameref <name>_to form (hot
path, writes a named variable) plus a thin printf wrapper <name> that
delegates to it. The leading "_" now exclusively marks private
helpers; the public escape API (called from sibling modules) is
unprefixed.

- current_date_text now delegates to current_date_text_to so both
  forms share the SHURIKEN_CURRENT_DATE_TEXT cache; the printf form no
  longer silently re-execs `date` on every direct call. Output
  unchanged.
- Drop the misleading "_" prefix on the public escape API and update
  all callers: _html_escape->html_escape, _css_string_escape->
  css_string_escape, _json_string->json_string, _json_bool->json_bool,
  _json_string_escape->json_string_escape.
- Add the missing JSON nameref forms: json_string_escape_to,
  json_string_to, json_bool_to (printf wrappers delegate to them).
- Add tests: JSON printf-vs-nameref parity and a current_date_text
  caching + nameref-parity check.

No escaping/encoding or date output changes -- API-shape/perf only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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