refactor(core): reuse read newline locator (#45616)

Reuse the reader newline locator for the terminal tree leaf while preserving accumulated offsets and the whole-tree fallback. Add chunk-boundary coverage.

packages/core/src/tool/read-filesystem.ts

@@ -361,12 +361,8 @@ const textOffset = (tree: TextNode, newline: number) => {
     if (!child) return tree.summary.bytes
     node = child
   }
-  for (const [index, byte] of node.bytes.entries()) {
-    if (byte !== 10) continue
-    remaining--
-    if (remaining === 0) return offset + index + 1
-  }
-  return tree.summary.bytes
+  const end = nthNewline(node.bytes, remaining)
+  return end === undefined ? tree.summary.bytes : offset + end
 }
 
 const nthNewline = (bytes: Uint8Array, count: number) => {

packages/core/test/tool-read-filesystem.test.ts

@@ -257,6 +257,21 @@ describe("ReadToolFileSystem", () => {
     }),
   )
 
+  it.effect("reads after a newline at the first chunk boundary", () =>
+    Effect.gen(function* () {
+      const { environment, files, directory } = yield* fixture
+      const file = path.join(directory, "boundary.txt")
+      yield* files.writeFileString(file, `${"a".repeat(256 * 1024 - 1)}\nsecond\n`)
+
+      const result = yield* ReadToolFileSystem.read(environment, absolute(file), "boundary.txt", {
+        offset: 2,
+        limit: 1,
+      })
+
+      expect(result).toMatchObject({ type: "text-page", content: "second", offset: 2, truncated: false })
+    }),
+  )
+
   it.effect("preserves the media ingestion limit message", () =>
     Effect.gen(function* () {
       const { environment, files, directory } = yield* fixture