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/fs/src/types.ts

@@ -101,6 +101,14 @@ export interface FsWriteOutcome {
   operation: 'create' | 'update'
   /** Opaque version of the file after the write. */
   version: FsVersion
+  /**
+   * The file's content BEFORE the write, or `null` when the file did not exist
+   * (a create). Raw storage text (LF-normalized by the backend), never a diff —
+   * a consumer computes the result-time contextual diff from `before`/`after`.
+   */
+  before: string | null
+  /** The file's content AFTER the write (the text that was written). */
+  after: string
 }
 
 /** A literal-replacement edit request. */
@@ -121,6 +129,14 @@ export interface FsEditOutcome {
   replaceAll: boolean
   /** Opaque version of the file after the edit. */
   version: FsVersion
+  /**
+   * The file's content BEFORE the edit. Raw storage text (LF-normalized by the
+   * backend), never a diff — a consumer computes the result-time contextual diff
+   * (the applied hunk with context) from `before`/`after`.
+   */
+  before: string
+  /** The file's content AFTER the edit. */
+  after: string
 }
 
 /**