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/dto.rs

@@ -1,3 +1,4 @@
+use crate::data_value::MetricPrimitiveKind;
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize)]
@@ -7,6 +8,14 @@ pub struct LogEntryDto {
     pub text: String,
 }
 
+#[derive(Debug, Clone, Serialize)]
+pub struct ParamDeclarationDto {
+    pub name: String,
+    pub message_match_regex: String,
+    pub capture_group: String,
+    pub kind: MetricPrimitiveKind,
+}
+
 #[derive(Debug, Clone, Serialize)]
 pub struct TimelineBucketDto {
     pub start_ms: i64,