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-local/src/fsio.ts

@@ -382,6 +382,25 @@ export async function readForEdit(
   return { content: normalizeLineEndings(raw), lineEndings: detectLineEndings(raw) }
 }
 
+/**
+ * Best-effort read of a file's current text for a before/after diff basis, used
+ * by an overwrite. Returns the LF-normalized decoded content, or `null` when the
+ * file is binary or not valid UTF-8 — a write must succeed regardless of the
+ * prior bytes, so an undiffable prior file simply yields no contextual diff
+ * (the caller treats `null` the same as an absent file: call-time card only).
+ */
+export async function readTextForDiff(absolutePath: string, signal?: AbortSignal): Promise<string | null> {
+  const buffer = await readFileAbortable(absolutePath, 'read', signal)
+  if (buffer.includes(0)) return null
+  try {
+    return normalizeLineEndings(new TextDecoder('utf-8', { fatal: true }).decode(buffer))
+  } catch (error: unknown) {
+    /* v8 ignore next 2 -- TextDecoder({fatal}) only throws TypeError on invalid bytes; any other throw is an unreachable runtime fault. */
+    if (!(error instanceof TypeError)) throw error
+    return null
+  }
+}
+
 /**
  * Apply a literal replacement to LF-normalized content. Throws
  * `FS_EDIT_NOT_FOUND` on empty `oldString` or zero matches and