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/core/tools/tests/tools.spec.ts

@@ -81,6 +81,38 @@ describe('ToolRegistry', () => {
     expect(result).toEqual({ callId: CallId('c1'), content: [{ type: 'text', text: 'hi' }], isError: false })
   })
 
+  it('threads a tool-attached meta (object return form) onto the result', async () => {
+    const ctx = await setup()
+    ctx.tools.register({
+      ...echoTool,
+      name: 'meta-tool',
+      async execute() {
+        return { content: [{ type: 'text', text: 'ok' }], meta: { diffs: [{ path: 'a', oldText: null, newText: 'x' }] } }
+      },
+    })
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'meta-tool', arguments: {} })
+    expect(result).toEqual({
+      callId: CallId('c1'),
+      content: [{ type: 'text', text: 'ok' }],
+      isError: false,
+      meta: { diffs: [{ path: 'a', oldText: null, newText: 'x' }] },
+    })
+  })
+
+  it('omits meta when the object return form supplies none', async () => {
+    const ctx = await setup()
+    ctx.tools.register({
+      ...echoTool,
+      name: 'no-meta-tool',
+      async execute() {
+        return { content: [{ type: 'text', text: 'ok' }] }
+      },
+    })
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'no-meta-tool', arguments: {} })
+    expect(result).toEqual({ callId: CallId('c1'), content: [{ type: 'text', text: 'ok' }], isError: false })
+    expect('meta' in result).toBe(false)
+  })
+
   it('returns isError results for unknown tools and throwing tools', async () => {
     const ctx = await setup()
     ctx.tools.register({