Extract param extraction into its own EngineSnapshot::ingest_params stage (ticket 101)

Raw storage, log-stream header derivation, and auto-param detection
were inlined directly in the ingest actor's write closure alongside
metric computation. Pulling them into their own method gives the
ingest arm two clearly separated, independently-callable stages —
param extraction (ingest_params) and metric computation
(apply_all_metrics) — with no behavior change, in preparation for
splitting these onto separate actors (ticket 102).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

src/engine/snapshot/mod.rs

@@ -45,6 +45,7 @@ use sort::{
     derive_single_param,
 };
 use std::collections::{BTreeMap, HashMap, HashSet};
+use std::sync::Arc;
 
 /// One row of a metric declaration: name, message-match regex, label param names, value param
 /// name, and the label-name-to-value-param and static-label maps.
@@ -812,6 +813,35 @@ impl EngineSnapshot {
             .count()
     }
 
+    /// The primary/param-extraction stage of ingesting one line: raw storage, log-stream header
+    /// derivation, and auto-param detection — deliberately excludes metric computation
+    /// (`apply_all_metrics`), which is the profile-scoped stage a caller applies separately, so
+    /// the two remain independently callable (ticket 100/101's precondition for eventually
+    /// running them on different actors, see `ideas/0001-actor-split.md`).
+    pub(crate) fn ingest_params(
+        &mut self,
+        sequence: u64,
+        timestamp: Timestamp,
+        text: &Arc<str>,
+        row_header: Option<&crate::engine::logfmt::LogStreamHeader>,
+    ) {
+        self.approx_bytes_total += text.approx_log_bytes();
+        self.index.insert(
+            IndexKey::IngestedLog {
+                sequence,
+                timestamp,
+            },
+            DataValue::LogLiteral {
+                text: Arc::clone(text),
+            },
+        );
+        self.log_count += 1;
+        if let Some(header) = row_header {
+            self.derive_log_stream_params_from_row(header, text);
+        }
+        self.auto_declare_params(text);
+    }
+
     pub(crate) fn auto_declare_params(&mut self, text: &str) {
         let mut total = self.auto_derived_param_count();
         if total >= MAX_AUTO_DECLARED_PARAMS {