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/diff.ts

@@ -0,0 +1,92 @@
+/**
+ * Result-time contextual-diff computation for the `write`/`edit` tools. Turns a
+ * before/after pair of file texts into one {@link FileDiff} per applied hunk —
+ * each hunk's `oldText`/`newText` reconstructed from the unified-diff lines with
+ * ±{@link DIFF_CONTEXT} surrounding context lines, matching how claude-agent-acp
+ * renders an editor inline diff.
+ *
+ * This is display-only presentation vocabulary (a UI concern), so it lives in
+ * the model-facing tool, NOT the `dsh-fs` storage seam — the backend returns
+ * only the raw before/after text (storage facts) and the tool computes the diff.
+ *
+ * @module @deepseek-ai/dsh-tool-fs/src/diff
+ */
+
+import { structuredPatch } from 'diff'
+import type { FileDiff } from '@deepseek-ai/dsh-tools'
+import type { JsonValue } from '@deepseek-ai/dsh-session'
+
+/** Context lines shown on each side of an applied hunk (matches claude-agent-acp). */
+export const DIFF_CONTEXT = 3
+
+/**
+ * The `write`/`edit` tools' private `tool/result` `meta` payload: the applied
+ * contextual-diff hunks. A {@link JsonValue} (persisted with the session log, so
+ * `presentResult` reproduces the diff card on replay). The producing tool owns
+ * this shape; the bridge only sees the opaque `meta` and the tool narrows it back
+ * via {@link diffsFromMeta}.
+ */
+export type FsDiffMeta = { diffs: FileDiff[] }
+
+/**
+ * Compute one {@link FileDiff} per hunk between `before` and `after`, each
+ * carrying the applied change plus {@link DIFF_CONTEXT} context lines. Returns an
+ * empty array when the texts are identical (no hunks). For a scattered
+ * `replace_all` edit the patch yields multiple hunks, so multiple `FileDiff`s
+ * come back — matching the editor rendering one diff block per site.
+ *
+ * Each hunk's `oldText` is its `-` (removed) and context lines joined by `\n`;
+ * `newText` is its `+` (added) and context lines. A hunk with no old lines
+ * (a pure insertion) reports `oldText: null` (nothing to diff against), mirroring
+ * the call-time card's new-file convention. The unified-diff "\ No newline at end
+ * of file" markers are dropped — they annotate the patch, not file content.
+ */
+export function computeHunkDiffs(path: string, before: string, after: string): FileDiff[] {
+  const patch = structuredPatch('', '', before, after, undefined, undefined, { context: DIFF_CONTEXT })
+  const diffs: FileDiff[] = []
+  for (const hunk of patch.hunks) {
+    const oldLines: string[] = []
+    const newLines: string[] = []
+    for (const line of hunk.lines) {
+      // The unified-diff marker for a missing trailing newline annotates the
+      // patch, not the content — skip it so it never leaks into a diff block.
+      if (line.startsWith('\\')) continue
+      const text = line.slice(1)
+      if (line.startsWith('-')) {
+        oldLines.push(text)
+      } else if (line.startsWith('+')) {
+        newLines.push(text)
+      } else {
+        // A context (unchanged) line appears on both sides.
+        oldLines.push(text)
+        newLines.push(text)
+      }
+    }
+    diffs.push({ path, oldText: oldLines.length > 0 ? oldLines.join('\n') : null, newText: newLines.join('\n') })
+  }
+  return diffs
+}
+
+/** Whether `value` is a valid {@link FileDiff} (defensive narrowing from opaque `meta`). */
+function isFileDiff(value: JsonValue): value is FileDiff & JsonValue {
+  if (typeof value !== 'object' || value === null || Array.isArray(value)) return false
+  const { path, oldText, newText } = value
+  return typeof path === 'string'
+    && (oldText === null || typeof oldText === 'string')
+    && typeof newText === 'string'
+}
+
+/**
+ * Narrow an opaque `tool/result` `meta` back to this tool's {@link FileDiff}
+ * hunks, or `undefined` when it is absent/malformed. `presentResult` runs on
+ * arbitrary logged `meta` (possibly from an older shape or a hand-edited log), so
+ * it validates defensively rather than trusting the payload — a bad `meta` yields
+ * no diff card (the generic result rendering) instead of a thrown presenter.
+ */
+export function diffsFromMeta(meta: JsonValue | undefined): FileDiff[] | undefined {
+  if (typeof meta !== 'object' || meta === null || Array.isArray(meta)) return undefined
+  const diffs = meta.diffs
+  if (!Array.isArray(diffs) || diffs.length === 0 || !diffs.every(isFileDiff)) return undefined
+  return diffs
+}
+