Implements the ParamDeclaration slice of the primary index: DataValue::ParamDeclaration
holds a regex + named capture group + MetricPrimitiveKind, stored under
IndexKey::ParamDeclaration{name} between LogParamSplit and ParamMetricSplit.
EngineMessage gains DeclareParam (validates the regex compiles and the capture
group exists before storing, returning a clear error otherwise) and
RemoveParamDeclaration; EngineQuery gains ParamDeclarations to list them back out
as ParamDeclarationDto. No API wiring yet (separate ticket).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o
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