fix(deps): cap placo below 0.9.16 and harden kinematics import (#3647)

* fix(deps): cap placo below 0.9.16 and harden kinematics import

placo 0.9.16 links against liburdfdom_sensor.so.4, which is unavailable
on Ubuntu 24.04 (noble ships urdfdom 3.x). Importing placo on that base
crashes with:

  ImportError: liburdfdom_sensor.so.4.0: cannot open shared object file

This broke nightly Latest Deps tests (CPU and GPU) when the lockfile
upgrade picked placo 0.9.16, since lerobot.model.kinematics
unconditionally imports placo when _placo_available is true, and that
check (importlib.util.find_spec) cannot detect dlopen failures of
transitive shared libraries — so unrelated subsystems (RL actor,
gym_manipulator) became unimportable.

Two changes:

1. Pin placo to <0.9.16 in pyproject.toml + regenerate uv.lock
   (0.9.16 → 0.9.15). Short-term unblock for nightly CI until system
   urdfdom 4.x is broadly available.

2. Harden the import guard in src/lerobot/model/kinematics.py:
   wrap 'import placo' in try/except ImportError so a missing
   transitive .so no longer crashes module import. RobotKinematics
   instantiation now raises an informative ImportError citing the
   underlying dlopen failure via _raise_if_placo_unusable().

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(kinematics): hoist _placo_runtime_error to module scope for mypy

Mypy walks the TYPE_CHECKING branch in which the runtime else-block is
not executed, so _placo_runtime_error was only defined at runtime and
mypy reported 'Name "_placo_runtime_error" is not defined' on the
three references inside _raise_if_placo_unusable. Declare the symbol
unconditionally at module scope with a default of None; the runtime
import-failure branch still assigns to it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* style(kinematics): drop verbose comments around placo import guard

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -138,7 +138,9 @@ dataset_viz = ["lerobot[dataset]", "lerobot[viz]"]
 # Common
 av-dep = ["av>=15.0.0,<16.0.0"]
 pygame-dep = ["pygame>=2.5.1,<2.7.0"]
-placo-dep = ["placo>=0.9.6,<0.9.17"]
+# NOTE: 0.9.16 links against liburdfdom_sensor.so.4, which is unavailable on Ubuntu 24.04
+# (noble ships urdfdom 3.x). Cap below 0.9.16 until system urdfdom 4.x is broadly available.
+placo-dep = ["placo>=0.9.6,<0.9.16"]
 transformers-dep = ["transformers>=5.4.0,<5.6.0"]
 grpcio-dep = ["grpcio==1.73.1", "protobuf>=6.31.1,<6.32.0"]
 can-dep = ["python-can>=4.2.0,<5.0.0"]