Secrets-009: SecretReferenceExpander.ExpandAsync throws FormatException when a value holds a
${secret: marker that is not a complete reference with a non-blank name (${secret:}, an
unterminated ${secret:name, a nested marker), instead of passing the literal text through as the
credential. ExpandConfigurationAsync names the offending key (never the value) and writes nothing.
The README wiring sample drops the relative "SqlitePath": "secrets.db" (which failed validation)
for the per-user default and shows an absolute example; a test validates the sample.
Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV
Live HistorianGateway boot caught that the expander scanned a '_secretsComment' appsettings
value documenting the ${secret:...} syntax, tried to resolve the literal example token, and
crashed boot. The family uses _*Comment keys pervasively, so skip any config key whose leaf
segment starts with '_' (the _comment convention). Functional keys never lead with '_'.
Joseph Doherty