feat: add file edit diff progress view

Capture file edit snapshots through runner tool lifecycle hooks and render unified diffs in the WebUI with folding and truncation controls.

webui/src/lib/file-diff.ts

@@ -0,0 +1,94 @@
+import { parsePatch } from "diff";
+
+import type { UIFileDiff } from "@/lib/types";
+
+export interface RenderableFileDiffLine {
+  kind: "context" | "add" | "delete";
+  old_lineno?: number | null;
+  new_lineno?: number | null;
+  content: string;
+}
+
+export interface RenderableFileDiffHunk {
+  old_start: number;
+  old_lines: number;
+  new_start: number;
+  new_lines: number;
+  lines: RenderableFileDiffLine[];
+}
+
+export interface RenderableFileDiff {
+  hunks: RenderableFileDiffHunk[];
+}
+
+export function hasRenderableFileDiff(diff?: UIFileDiff): boolean {
+  if (!diff) return false;
+  return typeof diff.text === "string" && diff.text.trim().length > 0;
+}
+
+export function parseRenderableFileDiff(diff: UIFileDiff): RenderableFileDiff {
+  if (typeof diff.text === "string" && diff.text.trim().length > 0) {
+    return parseUnifiedDiffText(diff.text);
+  }
+  return { hunks: [] };
+}
+
+function parseUnifiedDiffText(text: string): RenderableFileDiff {
+  let files: ReturnType<typeof parsePatch>;
+  try {
+    files = parsePatch(text);
+  } catch {
+    return { hunks: [] };
+  }
+  return {
+    hunks: files.flatMap((file) =>
+      file.hunks.map((hunk) => {
+        let oldLineno = hunk.oldStart;
+        let newLineno = hunk.newStart;
+        const lines: RenderableFileDiffLine[] = [];
+
+        for (const rawLine of hunk.lines) {
+          if (rawLine.startsWith("\\")) continue;
+          const marker = rawLine[0];
+          const content = rawLine.slice(1);
+          if (marker === "+") {
+            lines.push({
+              kind: "add",
+              old_lineno: null,
+              new_lineno: newLineno,
+              content,
+            });
+            newLineno += 1;
+            continue;
+          }
+          if (marker === "-") {
+            lines.push({
+              kind: "delete",
+              old_lineno: oldLineno,
+              new_lineno: null,
+              content,
+            });
+            oldLineno += 1;
+            continue;
+          }
+          lines.push({
+            kind: "context",
+            old_lineno: oldLineno,
+            new_lineno: newLineno,
+            content: marker === " " ? content : rawLine,
+          });
+          oldLineno += 1;
+          newLineno += 1;
+        }
+
+        return {
+          old_start: hunk.oldStart,
+          old_lines: hunk.oldLines,
+          new_start: hunk.newStart,
+          new_lines: hunk.newLines,
+          lines,
+        };
+      }),
+    ),
+  };
+}