feat: language annotation pipeline (#3471)

Steerable annotation pipeline (lerobot-annotate) that populates the language_persistent and language_events columns introduced in PR 1 (#3467) directly into data/chunk-*/file-*.parquet.

This is PR 2 of the three-PR plan:

PR 1 (Add extensive language support #3467): schema + DSL + rendering, base of this PR
PR 2 (this PR): annotation pipeline writing into PR 1's columns
PR 3: model with language prediction and runtime
A VLM (Qwen-VL family, served on vLLM) watches each episode's video and emits grounded language annotations: subtasks, plans, memory, task rephrasings, interjections + speech, and per-camera VQA. The pipeline is built for production annotation at scale — single-camera grounding, embedded-frame inputs, a describe-then-segment grounding flow, and a deterministic full-episode coverage guarantee — informed by Scale's dense-captioning findings (representation > sampling, rules > reasoning, model capacity is the biggest lever, two-pass systems compound errors)

tests/annotations/test_vlm_client.py

@@ -0,0 +1,41 @@
+#!/usr/bin/env python
+
+# Copyright 2026 The HuggingFace Inc. team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Unit tests for ``vlm_client`` helpers."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("datasets", reason="datasets is required (install lerobot[dataset])")
+
+from lerobot.annotations.steerable_pipeline.vlm_client import _bind_serve_port  # noqa: E402
+
+
+def test_bind_serve_port_substitutes_placeholder() -> None:
+    # The {port} placeholder is replaced everywhere it appears, regardless of
+    # parallel vs single server — the bug was the single-server path passing
+    # it through unsubstituted.
+    cmd = "vllm serve M --max-model-len 32768 --port {port}"
+    assert _bind_serve_port(cmd, 8000) == "vllm serve M --max-model-len 32768 --port 8000"
+
+
+def test_bind_serve_port_appends_when_missing() -> None:
+    assert _bind_serve_port("vllm serve M", 8001) == "vllm serve M --port 8001"
+
+
+def test_bind_serve_port_leaves_explicit_port_untouched() -> None:
+    cmd = "vllm serve M --port 9000"
+    assert _bind_serve_port(cmd, 8000) == cmd