Show the opposite trace endpoint

Why: The trace view selected an endpoint by lexical ID order and could present the current section as its own source.

Evidence: Focused presentation tests cover both selected directions of a source-to-section relationship.

Impact: Source and impact columns now identify the object opposite the selected node.

Refs: rdm-two-trees-latest-templates

src/components/RdmAuditWorkspace.tsx

@@ -7,6 +7,7 @@ import { Chip, Collapsible, EmptyState, SectionLabel } from '@ui'
 import { classifyRdmError } from '../rdmErrors'
 import {
   describeAuditEvent, describeTreeItem, humanizeKind, humanizeRelation, humanizeState, isTreeItemExpandable,
+  relationOtherEndpoint,
   type AuditEventViewInput,
 } from '../rdmAuditPresentation'
 import { store, useStore } from '../store'
@@ -478,11 +479,11 @@ function TraceTab({ pid }: { pid: string }) {
         <div className="rdm-audit-trace-cols">
           <div className="rdm-audit-trace-col">
             <SectionLabel>来源依据（{sources.length}）</SectionLabel>
-            {sources.length === 0 ? <div className="empty-note">无来源</div> : sources.map((r, i) => <RelationRow key={i} r={r} />)}
+            {sources.length === 0 ? <div className="empty-note">无来源</div> : sources.map((r, i) => <RelationRow key={i} r={r} selected={sel} />)}
           </div>
           <div className="rdm-audit-trace-col">
             <SectionLabel>影响对象（{impacts.length}）</SectionLabel>
-            {impacts.length === 0 ? <div className="empty-note">无影响</div> : impacts.map((r, i) => <RelationRow key={i} r={r} />)}
+            {impacts.length === 0 ? <div className="empty-note">无影响</div> : impacts.map((r, i) => <RelationRow key={i} r={r} selected={sel} />)}
           </div>
         </div>
       )}
@@ -491,8 +492,8 @@ function TraceTab({ pid }: { pid: string }) {
   )
 }
 
-function RelationRow({ r }: { r: RelationEntry }) {
-  const other = r.from?.id && r.to?.id ? (r.from?.id < r.to?.id ? r.from : r.to) : (r.from || r.to)
+function RelationRow({ r, selected }: { r: RelationEntry; selected: { kind: string; id: string } }) {
+  const other = relationOtherEndpoint(r, selected)
   const readable = humanizeRelation(r)
   return (
     <div className="rdm-audit-rel">

src/rdmAuditPresentation.test.ts

@@ -1,5 +1,8 @@
 import { describe, expect, it } from 'vitest'
-import { describeAuditEvent, describeTreeItem, humanizeKind, humanizeRelation, humanizeState, isTreeItemExpandable } from './rdmAuditPresentation'
+import {
+  describeAuditEvent, describeTreeItem, humanizeKind, humanizeRelation, humanizeState,
+  isTreeItemExpandable, relationOtherEndpoint,
+} from './rdmAuditPresentation'
 
 describe('RDM audit presentation', () => {
   it('turns a structured document event into a readable action and impact', () => {
@@ -74,4 +77,13 @@ describe('RDM audit presentation', () => {
       title: '模板内容 3',
     })
   })
+
+  it('shows the endpoint opposite the selected object in a trace relationship', () => {
+    const relation = {
+      from: { kind: 'slice', id: 'slc-6' },
+      to: { kind: 'section', id: 'sec-2' },
+    }
+    expect(relationOtherEndpoint(relation, { kind: 'section', id: 'sec-2' })).toEqual(relation.from)
+    expect(relationOtherEndpoint(relation, { kind: 'slice', id: 'slc-6' })).toEqual(relation.to)
+  })
 })

src/rdmAuditPresentation.ts

@@ -157,6 +157,15 @@ export function humanizeRelation(input: { layer?: string; type?: string; kind?:
   }
 }
 
+export function relationOtherEndpoint<T extends { kind?: string; id?: string }>(
+  relation: { from?: T; to?: T },
+  selected: { kind: string; id: string },
+): T | undefined {
+  if (relation.to?.kind === selected.kind && relation.to.id === selected.id) return relation.from
+  if (relation.from?.kind === selected.kind && relation.from.id === selected.id) return relation.to
+  return relation.from || relation.to
+}
+
 export function isTreeItemExpandable(item: { kind: string; hasChildren?: boolean; id?: string; title?: string }): boolean {
   return item.kind === 'doc' || item.hasChildren === true
 }