feat(tool-fs): result-time applied-hunk diffs for write/edit

fs write/edit now emit a result-time contextual-diff tool_call_update
(the applied hunk with ±3 context lines, one hunk per replace_all site),
matching what claude-agent-acp sends and what makes an editor render the
change in place. The call-time snippet diff stays; the result hunk
supersedes it (ACP content-replace).

Mechanism:
- A persisted tool-private `meta` channel: execute may return
  `{ content, meta }`; `meta` (JsonValue) rides on the tool/result event
  and is handed back to presentResult, so the diff reproduces on replay
  (event-sourced). JsonValue is now exported from dsh-session.
- The backend returns raw before/after text (storage facts) on
  FsWriteOutcome/FsEditOutcome; the tool computes the hunk via the npm
  `diff` package's structuredPatch. A create has no before → no result
  diff; a failed/aborted mutation carries no meta.
- ToolResultView gains a DiffResultView; the bridge's result-side switch
  renders it as {type:'diff'} content blocks.

RFC: docs/rfc/implemented/architecture/2026-07-02-result-time-applied-hunk-diffs.md
(justifies the npm `diff` runtime dep over vendoring and the meta channel);
the render-intent-union RFC's Non-goal is updated to record this shipped.
All fs snapshot goldens re-recorded; edit/overwrite gain the contextual
result diff, create/read/policy-reject unchanged in structure.

packages/fs/tool-fs/tests/tools.spec.ts

@@ -57,16 +57,17 @@ class FakeFs extends FileSystem {
   override async writeText(target: FsTarget, content: string, expected?: FsWriteIntent): Promise<FsWriteOutcome> {
     this.throwIfArmed()
     this.writeIntents.push(expected)
-    const existed = this.files.has(target.targetKey)
+    const before = this.files.get(target.targetKey) ?? null
     this.files.set(target.targetKey, content)
-    return { operation: existed ? 'update' : 'create', version: FsVersion('v2') }
+    return { operation: before !== null ? 'update' : 'create', version: FsVersion('v2'), before, after: content }
   }
   override async editText(target: FsTarget, edit: FsEditRequest, expected?: { version: FsVersion }): Promise<FsEditOutcome> {
     this.throwIfArmed()
     this.editIntents.push(expected)
     const content = this.files.get(target.targetKey) ?? ''
-    this.files.set(target.targetKey, content.split(edit.oldString).join(edit.newString))
-    return { replacements: 1, replaceAll: edit.replaceAll, version: FsVersion('v3') }
+    const after = content.split(edit.oldString).join(edit.newString)
+    this.files.set(target.targetKey, after)
+    return { replacements: 1, replaceAll: edit.replaceAll, version: FsVersion('v3'), before: content, after }
   }
 }
 
@@ -395,3 +396,80 @@ describe('tool-owned presentation (pure presentCall)', () => {
     })
   })
 })
+
+describe('result-time contextual diff (meta + presentResult)', () => {
+  // An edit records the applied contextual hunk on `tool/result` meta, and the
+  // tool's presentResult narrows it back into a `diff` result card the bridge
+  // renders. Drive execute end-to-end so the meta is the REAL computed hunk.
+  const withContext = 'a\nb\nc\nOLD\nd\ne\nf\n'
+
+  it('edit: execute attaches the applied hunk as meta { diffs }', async () => {
+    const { ctx, fs } = await setup()
+    const session = { header: {} }
+    fs.files.set('key:a.txt', withContext)
+    await call(ctx, 'read', { file_path: 'a.txt' }, { session })
+    const result = await call(ctx, 'edit', { file_path: 'a.txt', old_string: 'OLD', new_string: 'NEW' }, { session })
+    expect(result.isError).toBe(false)
+    expect(result.meta).toEqual({
+      diffs: [{ path: 'a.txt', oldText: 'a\nb\nc\nOLD\nd\ne\nf', newText: 'a\nb\nc\nNEW\nd\ne\nf' }],
+    })
+  })
+
+  it('edit: presentResult turns the meta into a diff result card', async () => {
+    const { ctx, fs } = await setup()
+    const session = { header: {} }
+    fs.files.set('key:a.txt', withContext)
+    await call(ctx, 'read', { file_path: 'a.txt' }, { session })
+    const result = await call(ctx, 'edit', { file_path: 'a.txt', old_string: 'OLD', new_string: 'NEW' }, { session })
+    const view = ctx.tools.get('edit')?.presentResult?.({ file_path: 'a.txt', old_string: 'OLD', new_string: 'NEW' }, result)
+    expect(view).toEqual({
+      card: 'diff', title: 'Edit a.txt',
+      diffs: [{ path: 'a.txt', oldText: 'a\nb\nc\nOLD\nd\ne\nf', newText: 'a\nb\nc\nNEW\nd\ne\nf' }],
+    })
+  })
+
+  it('write OVERWRITE: execute attaches a contextual hunk; presentResult renders a diff card', async () => {
+    const { ctx, fs } = await setup()
+    const session = { header: {} }
+    fs.files.set('key:a.txt', withContext)
+    await call(ctx, 'read', { file_path: 'a.txt' }, { session })
+    const result = await call(ctx, 'write', { file_path: 'a.txt', content: 'a\nb\nc\nNEW\nd\ne\nf\n' }, { session })
+    expect(result.isError).toBe(false)
+    expect(result.meta).toEqual({ diffs: [{ path: 'a.txt', oldText: 'a\nb\nc\nOLD\nd\ne\nf', newText: 'a\nb\nc\nNEW\nd\ne\nf' }] })
+    const view = ctx.tools.get('write')?.presentResult?.({ file_path: 'a.txt', content: 'x' }, result)
+    expect(view).toEqual({ card: 'diff', title: 'Write a.txt', diffs: [{ path: 'a.txt', oldText: 'a\nb\nc\nOLD\nd\ne\nf', newText: 'a\nb\nc\nNEW\nd\ne\nf' }] })
+  })
+
+  it('write CREATE: no before-version → no meta, presentResult returns undefined (call-time card stands)', async () => {
+    const { ctx } = await setup()
+    const session = { header: {} }
+    const result = await call(ctx, 'write', { file_path: 'new.txt', content: 'fresh\n' }, { session })
+    expect(result.isError).toBe(false)
+    expect(result.meta).toBeUndefined()
+    expect(ctx.tools.get('write')?.presentResult?.({ file_path: 'new.txt', content: 'fresh\n' }, result)).toBeUndefined()
+  })
+
+  it('write OVERWRITE with identical content: a before exists but yields no hunk → no meta', async () => {
+    const { ctx, fs } = await setup()
+    const session = { header: {} }
+    fs.files.set('key:a.txt', 'same\n')
+    await call(ctx, 'read', { file_path: 'a.txt' }, { session })
+    const result = await call(ctx, 'write', { file_path: 'a.txt', content: 'same\n' }, { session })
+    expect(result.isError).toBe(false)
+    expect(result.meta).toBeUndefined()
+  })
+
+  it('presentResult returns undefined on an error result (nothing applied)', async () => {
+    const { ctx } = await setup()
+    const errorResult = { content: [{ type: 'text' as const, text: 'Error: boom' }], isError: true }
+    expect(ctx.tools.get('edit')?.presentResult?.({ file_path: 'a.txt', old_string: 'x', new_string: 'y' }, errorResult)).toBeUndefined()
+    expect(ctx.tools.get('write')?.presentResult?.({ file_path: 'a.txt', content: 'y' }, errorResult)).toBeUndefined()
+  })
+
+  it('presentResult returns undefined on malformed meta (defensive narrowing)', async () => {
+    const { ctx } = await setup()
+    const badMeta = { content: [{ type: 'text' as const, text: 'ok' }], isError: false, meta: { diffs: 'nope' } }
+    expect(ctx.tools.get('edit')?.presentResult?.({ file_path: 'a.txt', old_string: 'x', new_string: 'y' }, badMeta)).toBeUndefined()
+    expect(ctx.tools.get('write')?.presentResult?.({ file_path: 'a.txt', content: 'y' }, badMeta)).toBeUndefined()
+  })
+})