fix(app): make composer track clearance deterministic

Layout measurement excluded pill margin on some renderers, collapsing the intended gap above visible tracks. Keep the two-pill rail on one line and derive its complete tail clearance from shared geometry so chat switching and layout callbacks cannot change the result.

packages/app/src/agent-stream/bottom-overlay-inset.test.ts

@@ -5,8 +5,7 @@ describe("resolveBottomOverlayTailInset", () => {
   it("adds only the space missing after existing footer clearance", () => {
     expect(
       resolveBottomOverlayTailInset({
-        overlayHeight: 44,
-        clearance: 12,
+        requiredTailClearance: 56,
         existingTailSpacing: 24,
       }),
     ).toBe(32);
@@ -15,8 +14,7 @@ describe("resolveBottomOverlayTailInset", () => {
   it("reserves the full overlay and clearance when the tail has no spacing", () => {
     expect(
       resolveBottomOverlayTailInset({
-        overlayHeight: 44,
-        clearance: 12,
+        requiredTailClearance: 56,
         existingTailSpacing: 0,
       }),
     ).toBe(56);

packages/app/src/agent-stream/bottom-overlay-inset.ts

@@ -1,11 +1,9 @@
 export function resolveBottomOverlayTailInset({
-  overlayHeight,
-  clearance,
+  requiredTailClearance,
   existingTailSpacing,
 }: {
-  overlayHeight: number;
-  clearance: number;
+  requiredTailClearance: number;
   existingTailSpacing: number;
 }): number {
-  return Math.max(0, overlayHeight + clearance - existingTailSpacing);
+  return Math.max(0, requiredTailClearance - existingTailSpacing);
 }

packages/app/src/agent-stream/view.tsx

@@ -256,10 +256,8 @@ export interface AgentStreamViewProps {
   turnPresentation: TurnPresentation;
   routeBottomAnchorRequest?: BottomAnchorRouteRequest | null;
   isAuthoritativeHistoryReady?: boolean;
-  /** Measured transparent overlay at the bottom edge, rendered by the owning panel. */
-  bottomOverlayHeight?: number;
-  /** Visible space kept between the final content and that overlay. */
-  bottomOverlayClearance?: number;
+  /** Tail space required by a transparent overlay rendered at the bottom edge. */
+  bottomOverlayTailClearance?: number;
   toast?: ToastApi | null;
   onOpenWorkspaceFile?: (request: WorkspaceFileOpenRequest) => void;
   readOnly?: boolean;
@@ -308,8 +306,7 @@ const AgentStreamViewComponent = forwardRef<AgentStreamViewHandle, AgentStreamVi
       turnPresentation,
       routeBottomAnchorRequest = null,
       isAuthoritativeHistoryReady = true,
-      bottomOverlayHeight = 0,
-      bottomOverlayClearance = 0,
+      bottomOverlayTailClearance = 0,
       toast,
       onOpenWorkspaceFile,
       readOnly = false,
@@ -966,8 +963,7 @@ const AgentStreamViewComponent = forwardRef<AgentStreamViewHandle, AgentStreamVi
       const existingTailSpacing =
         auxiliary.turnFooter && !auxiliary.pendingPermissions ? TURN_FOOTER_BOTTOM_SPACING : 0;
       const bottomOverlayInset = resolveBottomOverlayTailInset({
-        overlayHeight: bottomOverlayHeight,
-        clearance: bottomOverlayClearance,
+        requiredTailClearance: bottomOverlayTailClearance,
         existingTailSpacing,
       });
       return renderLiveAuxiliaryNode({
@@ -975,12 +971,7 @@ const AgentStreamViewComponent = forwardRef<AgentStreamViewHandle, AgentStreamVi
         turnFooter: auxiliary.turnFooter,
         bottomOverlayInset,
       });
-    }, [
-      auxiliary.pendingPermissions,
-      auxiliary.turnFooter,
-      bottomOverlayClearance,
-      bottomOverlayHeight,
-    ]);
+    }, [auxiliary.pendingPermissions, auxiliary.turnFooter, bottomOverlayTailClearance]);
 
     const renderers = useMemo<StreamSegmentRenderers>(
       () => ({