perf: eliminate unnecessary clones and pre-allocate collections

Three micro-optimizations with zero behavioral change:

1. timeline_collect.rs: Reorder format!() before enum construction so
   the owned String moves into the variant directly, eliminating
   .clone() on state, label, and milestone strings in StateChanged,
   LabelAdded/Removed, and MilestoneSet/Removed event paths.

2. pipeline.rs: Use Arc<str> for doc_hash shared across a document's
   chunks instead of cloning the full String per chunk. Also remove
   redundant embed_buf.reserve() since extend_from_slice already
   handles growth and the buffer is reused across iterations.

3. rrf.rs: Pre-allocate HashMap with combined vector+fts result count
   via with_capacity() to avoid rehashing during RRF score accumulation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core/timeline_collect.rs

@@ -146,16 +146,15 @@ fn collect_state_events(
             continue;
         }
 
+        let summary = format!("State changed to {state}");
         events.push(TimelineEvent {
             timestamp: created_at,
             entity_type: entity.entity_type.clone(),
             entity_id: entity.entity_id,
             entity_iid: entity.entity_iid,
             project_path: entity.project_path.clone(),
-            event_type: TimelineEventType::StateChanged {
-                state: state.clone(),
-            },
-            summary: format!("State changed to {state}"),
+            event_type: TimelineEventType::StateChanged { state },
+            summary,
             actor,
             url: None,
             is_seed,
@@ -195,18 +194,14 @@ fn collect_label_events(
         let label = label_name.unwrap_or_else(|| "[deleted label]".to_owned());
 
         let (event_type, summary) = match action.as_str() {
-            "add" => (
-                TimelineEventType::LabelAdded {
-                    label: label.clone(),
-                },
-                format!("Label added: {label}"),
-            ),
-            "remove" => (
-                TimelineEventType::LabelRemoved {
-                    label: label.clone(),
-                },
-                format!("Label removed: {label}"),
-            ),
+            "add" => {
+                let summary = format!("Label added: {label}");
+                (TimelineEventType::LabelAdded { label }, summary)
+            }
+            "remove" => {
+                let summary = format!("Label removed: {label}");
+                (TimelineEventType::LabelRemoved { label }, summary)
+            }
             _ => continue,
         };
 
@@ -257,18 +252,14 @@ fn collect_milestone_events(
         let milestone = milestone_title.unwrap_or_else(|| "[deleted milestone]".to_owned());
 
         let (event_type, summary) = match action.as_str() {
-            "add" => (
-                TimelineEventType::MilestoneSet {
-                    milestone: milestone.clone(),
-                },
-                format!("Milestone set: {milestone}"),
-            ),
-            "remove" => (
-                TimelineEventType::MilestoneRemoved {
-                    milestone: milestone.clone(),
-                },
-                format!("Milestone removed: {milestone}"),
-            ),
+            "add" => {
+                let summary = format!("Milestone set: {milestone}");
+                (TimelineEventType::MilestoneSet { milestone }, summary)
+            }
+            "remove" => {
+                let summary = format!("Milestone removed: {milestone}");
+                (TimelineEventType::MilestoneRemoved { milestone }, summary)
+            }
             _ => continue,
         };