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.

docs/core-data-structures/tools.md

@@ -10,7 +10,7 @@ A `ToolSchema` (the model-facing fields) plus the `execute` function and optiona
 
 ```ts type-equiv
 interface ToolDefinition extends ToolSchema {
-  execute(args: unknown, exec: ToolExecution): Promise<ContentBlock[]>
+  execute(args: unknown, exec: ToolExecution): Promise<ToolExecuteReturn>
   /**
    * Optional: how to present the PENDING state of one call in a UI, derived from
    * the call's `args` (parsed arguments, `unknown` — the tool validates/narrows
@@ -100,6 +100,13 @@ interface ToolExecutionResult {
    * text in `content` is always present; this is extra structure for code.
    */
   error?: ToolErrorInfo
+  /**
+   * The tool-private presentation payload from a successful `execute` (the object
+   * return form). Threaded onto the `tool/result` session event and back into
+   * {@link ToolResult} for `presentResult`. Opaque {@link JsonValue}; absent when
+   * the tool attached none or the call failed.
+   */
+  meta?: JsonValue
 }
 ```