fix(clippy): Rust 1.95 compatibility — sort_by_key + collapsible_match

CI runs on stable Rust which advanced from 1.93 to 1.95, introducing
two newly-enforced lints under -D warnings:

- clippy::unnecessary_sort_by (12 sites) — rewrite sort_by closures
  that only call .cmp on Copy keys as sort_by_key, using std::cmp::Reverse
  for the descending ones.
- clippy::collapsible_match (1 site) — merge the MemoryState::Dormant
  inner if into a match guard. The equivalent rewrite for the Ping/Pong
  arm in websocket.rs is blocked by a move of non-Copy Bytes across the
  match-guard boundary, so that one site gets an explicit #[allow].

Files touched:
- crates/vestige-core/src/advanced/{chains,compression,cross_project,reconsolidation}.rs
- crates/vestige-core/src/codebase/{git,relationships}.rs
- crates/vestige-core/src/neuroscience/{importance_signals,memory_states,predictive_retrieval}.rs
- crates/vestige-mcp/src/tools/{changelog,cross_reference}.rs
- crates/vestige-mcp/src/dashboard/websocket.rs

Verified: cargo clippy --workspace -- -D warnings green on rustc 1.95.0;
cargo test --workspace unchanged (1292 tests green).

crates/vestige-core/src/codebase/git.rs

@@ -673,11 +673,11 @@ impl GitAnalyzer {
 
         // Top contributors
         let mut top_contributors: Vec<_> = author_counts.into_iter().collect();
-        top_contributors.sort_by(|a, b| b.1.cmp(&a.1));
+        top_contributors.sort_by_key(|b| std::cmp::Reverse(b.1));
 
         // Hot files (most frequently changed)
         let mut hot_files: Vec<_> = file_counts.into_iter().collect();
-        hot_files.sort_by(|a, b| b.1.cmp(&a.1));
+        hot_files.sort_by_key(|b| std::cmp::Reverse(b.1));
 
         Ok(HistoryAnalysis {
             bug_fixes,

crates/vestige-core/src/codebase/relationships.rs

@@ -579,7 +579,7 @@ impl RelationshipTracker {
         }
 
         let mut sorted: Vec<_> = file_degrees.into_iter().collect();
-        sorted.sort_by(|a, b| b.1.cmp(&a.1));
+        sorted.sort_by_key(|b| std::cmp::Reverse(b.1));
         sorted.truncate(limit);
 
         sorted