add workspace grep engine + search.grep/cancel, editor.open --line

The pure-Dart content-search engine behind find-in-files (D-79): walks
the ignore-pruned workspace, fans files across worker isolates
(Isolate.run) for parallelism, matches each line with a literal
indexOf fast-path or a RegExp, and streams match batches with
cooperative cancellation. No ripgrep dependency; the search.grep IPC
contract is engine-agnostic so an rg accelerator can slot in later.

search.grep returns a searchId and streams search.match / search.done
(or search.error) events, mirroring files.watch; search.cancel stops
an in-flight search. The service reuses the files service's resolved
ignore set so both honour the same ignore_files: layering.

editor.open gains an optional 1-based line argument: it converts the
line to a byte offset and sets the initial selection, enabling
click-to-line from search results.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/src/daemon/editor_commands.dart

@@ -11,6 +11,7 @@ library;
 
 import 'dart:io' show FileSystemException;
 
+import '../editor/buffer.dart' show Selection;
 import '../editor/registry.dart';
 import '../ipc/envelope.dart';
 import '../ipc/errno_mapping.dart';
@@ -66,6 +67,14 @@ Future<IpcResponse> _open(IpcRequest req, EditorRegistry r) async {
   }
   try {
     final buf = await r.open(path);
+    // Optional 1-based line: jump the initial selection to that line's
+    // start (used by find-in-files click-to-line, T-52). Out-of-range
+    // lines clamp via setSelection. A non-numeric/<1 line is ignored.
+    final rawLine = req.args['line'];
+    final line = rawLine is num ? rawLine.toInt() : int.tryParse('$rawLine');
+    if (line != null && line >= 1) {
+      r.setSelection(buf.id, Selection.collapsed(_offsetForLine(buf.content, line)));
+    }
     return IpcResponse.ok(id: req.id, data: buf.toJson());
   } on FileSystemException catch (e) {
     final errno = e.osError?.errorCode;
@@ -95,6 +104,19 @@ Future<IpcResponse> _open(IpcRequest req, EditorRegistry r) async {
   }
 }
 
+/// Byte offset of the start of the 1-based [line] in [content]. Lines
+/// past the end clamp to the content length.
+int _offsetForLine(String content, int line) {
+  if (line <= 1) return 0;
+  var remaining = line - 1;
+  var i = 0;
+  while (i < content.length && remaining > 0) {
+    if (content.codeUnitAt(i) == 0x0A) remaining--;
+    i++;
+  }
+  return i;
+}
+
 Future<IpcResponse> _active(IpcRequest req, EditorRegistry r) async {
   final buf = r.active;
   if (buf == null) {