Add param declaration storage and validation to engine index

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

src/data_value.rs

@@ -25,6 +25,12 @@ pub enum MetricPrimitiveKind {
 pub enum DataValue {
     /// Paired with `IndexKey::IngestedLog`.
     LogLiteral { text: String },
+    /// Paired with `IndexKey::ParamDeclaration`.
+    ParamDeclaration {
+        message_match_regex: String,
+        capture_group: String,
+        kind: MetricPrimitiveKind,
+    },
     /// Paired with `IndexKey::MetricDeclaration`.
     MetricDeclaration {
         message_match_regex: String,