URLs like "{unsubscribe}" parse without error in Go, so they were
reported as valid links and even counted as unsubscribe methods just
because they contain the word "unsubscribe".
Detect common un-substituted merge-field syntaxes ({x}, {{x}}, ${x},
*|X|*, %x%, [x], %7Bx%7D), mark such links invalid (Broken status),
exclude them from unsubscribe method detection, and surface them via a
new unreplaced_template content issue.
