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/session/src/json.ts

@@ -13,6 +13,16 @@
  * @module @deepseek-ai/dsh-session/json
  */
 
+/**
+ * A value that round-trips losslessly through JSON: `null`, a boolean, a finite
+ * number, a string, an array of such values, or a plain object whose values are
+ * such values. The static type companion to {@link isJsonValue} (which validates
+ * the same shape at runtime). Use it to type a payload that must survive
+ * session-log persistence and replay byte-identically — e.g. a tool's private
+ * presentation `meta`.
+ */
+export type JsonValue = null | boolean | number | string | JsonValue[] | { [key: string]: JsonValue }
+
 /**
  * Whether `value` is losslessly JSON-serializable: only `null`, finite numbers,
  * booleans, strings, plain arrays, and plain objects of such values. Rejects