Split src/ into an api/cli/engine/config module hierarchy

- src/api/: HTTP router + DTOs + the MCP server (the HTTP/MCP-facing
  surface): mod.rs (formerly api.rs), dto.rs, mcp_server.rs.
- src/cli/: argument parsing and process entry (formerly inline in
  main.rs) plus session registry: mod.rs, sessions.rs. main.rs is now
  a thin entry point that just declares modules and calls cli::run().
- src/engine/: the actor + everything it depends on to parse/index/
  query log lines: mod.rs (formerly engine.rs), alerts.rs, promql.rs,
  rate.rs, ingest.rs, logfmt.rs, filter_parser.rs/.pest,
  filter_datetime.rs, index_key.rs, data_value.rs, metric_primitive.rs,
  timestamp.rs.
- src/config/: persisted configuration: mod.rs (formerly settings.rs),
  profile.rs.
- src/consts.rs stays at the crate root, used from every module above.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A1ZJ3SrayzLzVndhVq9BmS

src/engine/index_key.rs

@@ -0,0 +1,32 @@
+use crate::engine::metric_primitive::MetricPrimitive;
+use crate::engine::timestamp::Timestamp;
+use serde::Serialize;
+use std::collections::BTreeMap;
+
+#[allow(dead_code)]
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Serialize)]
+pub enum IndexKey {
+    Start,
+    IngestedLog {
+        sequence: u64,
+        timestamp: Timestamp,
+    },
+    LogParamSplit,
+    ParamDeclaration {
+        name: String,
+    },
+    ParamMetricSplit,
+    MetricDeclaration {
+        name: String,
+    },
+    MetricBucketSplit,
+    Bucket {
+        metric_name: String,
+        labels: BTreeMap<String, MetricPrimitive>,
+    },
+    LastMetricEvent {
+        metric_name: String,
+        labels: BTreeMap<String, MetricPrimitive>,
+    },
+    End,
+}