fix(history): preserve branch colors across parent lanes

Keep branch metadata attached to each parent lane so the history graph
can render consistent colors and markers after merges and branch
transitions. Also soften the styling for behind-path segments to better
match the updated dot coloring.

src/lib/components/HistoryPanel.svelte

@@ -140,15 +140,18 @@
 
       const currentBranches = branchMembership.get(commit.hash) ?? localBranchRefs(commit);
       const commitBranches = uniqueStrings([...(beforeBranches[col] ?? []), ...currentBranches]);
-      after[col] = commit.parents.length > 0 ? commit.parents[0] : null;
-      afterBranches[col] = after[col] ? commitBranches.slice() : [];
+      const firstParent = commit.parents[0] ?? null;
+      after[col] = firstParent;
+      afterBranches[col] = firstParent
+        ? (branchMembership.get(firstParent) ?? commitBranches).slice()
+        : [];
 
       const fromCommit = new Set<number>([col]);
       for (let p = 1; p < commit.parents.length; p++) {
         let slot = after.indexOf(null);
         if (slot === -1) { slot = after.length; after.push(null); afterBranches.push([]); }
         after[slot] = commit.parents[p];
-        afterBranches[slot] = [];
+        afterBranches[slot] = (branchMembership.get(commit.parents[p]) ?? commitBranches).slice();
         fromCommit.add(slot);
       }