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/rfc/README.md

@@ -126,6 +126,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 | [Make `dsh-fs-policy` an event-gate plugin, not a method interface](implemented/architecture/2026-06-26-file-context-as-event-gate.md) | 2026-06-26 |
 | [Resolve filesystem paths against the caller's session cwd](implemented/architecture/2026-07-02-fs-per-session-cwd.md) | 2026-07-02 |
 | [Tagged render-intent union for tool-call presentation](implemented/architecture/2026-07-02-tool-render-intent-union.md) | 2026-07-02 |
+| [Result-time applied-hunk diffs for file mutations](implemented/architecture/2026-07-02-result-time-applied-hunk-diffs.md) | 2026-07-02 |
 
 ### Process