FN-5666: exclude base commit from done-task diff ranges

Prevent done-task diff endpoints from counting base-commit-only lineage as task-owned changes.

- add base-boundary handling in done-task merge SHA resolution so `baseCommitSha` is excluded by default and only consulted for boundary checks
- short-circuit `/diff` and `/file-diffs` responses to empty results when resolved done-task SHA is at or below `baseCommitSha`
- export and use an ancestry helper to detect base-boundary cases, while preserving normal and rebase range behavior
- extend done-task diff route tests with FN-5666 cases for no-op merge SHA, lineage association at base commit, normal post-base commits, and rebase ranges
- add a patch changeset for `@runfusion/fusion`

Files changed:
 .changeset/fn-5666-base-commit-exclusive.md        |   5 +
 .../src/__tests__/routes-diff-done-tasks.test.ts   | 217 ++++++++++++++++++++-
 .../src/routes/register-session-diff-routes.ts     |  57 +++++-
 3 files changed, 273 insertions(+), 6 deletions(-)

Fusion-Task-Id: FN-5666
Fusion-Task-Lineage: 782f9875-856a-4d31-9505-0bcd638c9c29

packages/dashboard/src/routes/register-session-diff-routes.ts

@@ -378,17 +378,43 @@ async function resolveAuditCommitSha(taskId: string, scopedStore: DoneTaskAggreg
   return undefined;
 }
 
-async function resolveDoneTaskMergeSha(task: DoneTaskAggregationTask, scopedStore: DoneTaskAggregationStore): Promise<string | undefined> {
+export async function isAtOrBelowTaskBase(sha: string, baseCommitSha: string, rootDir: string): Promise<boolean> {
+  if (sha === baseCommitSha) return true;
+  try {
+    await runGitCommand(["merge-base", "--is-ancestor", sha, baseCommitSha], rootDir, 5000);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+async function resolveDoneTaskMergeSha(
+  task: DoneTaskAggregationTask & { baseCommitSha?: string | null },
+  scopedStore: DoneTaskAggregationStore,
+  options?: { includeBaseCommitSha?: boolean },
+): Promise<string | undefined> {
+  const rootDir = scopedStore.getRootDir();
+  const baseCommitSha = task.baseCommitSha?.trim();
+  const includeBaseCommitSha = options?.includeBaseCommitSha === true;
+
   const existing = task.mergeDetails?.commitSha?.trim();
-  if (existing) return existing;
+  if (existing) {
+    if (!includeBaseCommitSha && baseCommitSha && existing === baseCommitSha) return undefined;
+    return existing;
+  }
 
   const auditSha = await resolveAuditCommitSha(task.id, scopedStore);
-  if (auditSha) return auditSha;
+  if (auditSha) {
+    if (!includeBaseCommitSha && baseCommitSha && auditSha === baseCommitSha) return undefined;
+    return auditSha;
+  }
 
   if (!task.lineageId) return undefined;
   const associations = await scopedStore.getTaskCommitAssociationsByLineageId(task.lineageId);
   for (const association of associations) {
-    if (association.commitSha && (await isReachableFromHead(association.commitSha, scopedStore.getRootDir()))) {
+    if (!association.commitSha) continue;
+    if (!includeBaseCommitSha && baseCommitSha && association.commitSha === baseCommitSha) continue;
+    if (await isReachableFromHead(association.commitSha, rootDir)) {
       return association.commitSha;
     }
   }
@@ -670,7 +696,19 @@ export function registerSessionDiffRoutes(router: Router, deps: SessionDiffRoute
       }
 
       if (task.column === "done") {
+        const mergeShaForBaseBoundary = await resolveDoneTaskMergeSha(task, scopedStore, { includeBaseCommitSha: true });
         const resolvedMergeSha = await resolveDoneTaskMergeSha(task, scopedStore);
+        if (mergeShaForBaseBoundary && task.baseCommitSha) {
+          // FN-5666: `git diff A..B` is exclusive of A, and baseCommitSha is the
+          // task fork point, so when resolved SHA is at/below base the task has
+          // no owned changes to display.
+          const atOrBelowBase = await isAtOrBelowTaskBase(mergeShaForBaseBoundary, task.baseCommitSha, scopedStore.getRootDir());
+          if (atOrBelowBase) {
+            res.json({ files: [], stats: { filesChanged: 0, additions: 0, deletions: 0 } });
+            return;
+          }
+        }
+
         const doneTaskForDiff = resolvedMergeSha
           ? {
               ...task,
@@ -931,7 +969,18 @@ export function registerSessionDiffRoutes(router: Router, deps: SessionDiffRoute
       }
 
       if (task.column === "done") {
+        const mergeShaForBaseBoundary = await resolveDoneTaskMergeSha(task, scopedStore, { includeBaseCommitSha: true });
         const resolvedMergeSha = await resolveDoneTaskMergeSha(task, scopedStore);
+        if (mergeShaForBaseBoundary && task.baseCommitSha) {
+          // FN-5666: baseCommitSha is the branch fork point and must remain
+          // exclusive in per-task display diffs.
+          const atOrBelowBase = await isAtOrBelowTaskBase(mergeShaForBaseBoundary, task.baseCommitSha, scopedStore.getRootDir());
+          if (atOrBelowBase) {
+            res.json([]);
+            return;
+          }
+        }
+
         const doneTaskForDiff = resolvedMergeSha
           ? {
               ...task,