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>

src/lerobot/model/kinematics.py

@@ -18,12 +18,25 @@ from typing import TYPE_CHECKING
 
 import numpy as np
 
-from lerobot.utils.import_utils import _placo_available, require_package
+from lerobot.utils.import_utils import require_package
 
-if TYPE_CHECKING or _placo_available:
+_placo_runtime_error: ImportError | None = None
+
+if TYPE_CHECKING:
     import placo  # type: ignore[import-not-found]
 else:
-    placo = None
+    try:
+        import placo  # type: ignore[import-not-found]
+    except ImportError as _placo_import_err:
+        placo = None
+        _placo_runtime_error = _placo_import_err
+
+
+def _raise_if_placo_unusable() -> None:
+    if placo is None and _placo_runtime_error is not None:
+        raise ImportError(
+            f"placo is installed but failed to import: {_placo_runtime_error!s}"
+        ) from _placo_runtime_error
 
 
 class RobotKinematics:
@@ -44,6 +57,7 @@ class RobotKinematics:
             joint_names (list[str] | None): List of joint names to use for the kinematics solver
         """
         require_package("placo", extra="placo-dep")
+        _raise_if_placo_unusable()
 
         self.robot = placo.RobotWrapper(urdf_path)
         self.solver = placo.KinematicsSolver(self.robot)