Sentinel-substitution approach: unresolved {{x.y.z}} tokens are replaced
with a unique NUL-delimited sentinel before Mustache compilation, rendered
as opaque text, then post-replaced with the original {{x.y.z}} literal.
Malformed templates (unclosed {{) are caught and return the raw template.
Never throws. 9 unit tests.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>