New node that fills a template string containing {placeholder} tokens
from up to 5 key/value pairs wired from upstream nodes. Unknown
placeholders are left intact rather than erroring, so partial templates
work during workflow development. Malformed templates (unclosed braces)
return the original string unchanged.
12 tests added covering substitution, passthrough, edge cases,
and malformed input.
https://claude.ai/code/session_01JBWyFr5shvPXUPmZkX3Nhg
