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/tool-fs/src/index.ts

@@ -32,6 +32,8 @@ export { applyWriteTool, formatWriteOutput, parseWriteArgs } from './write.ts'
 export { applyEditTool, formatEditOutput, parseEditArgs } from './edit.ts'
 export { READ_MAX_BYTES, READ_MAX_LINE_LENGTH, buildWindow, formatReadOutput } from './read-render.ts'
 export type { FileReadOutcome, FileTextLine, ReadWindow, WindowResult } from './read-render.ts'
+export { DIFF_CONTEXT, computeHunkDiffs, diffsFromMeta } from './diff.ts'
+export type { FsDiffMeta } from './diff.ts'
 
 /** Cordis plugin name used by loader diagnostics. */
 export const name = 'tool-fs'