fix: exports crash without optional deps, Vector.like() type safety, and docstring corrections

[henryiii]

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Summary

Part of #711

• Fix from vector import * crash without optional deps (src/vector/__init__.py): All optional names (MomentumSympy2D/3D/4D, VectorSympy2D/3D/4D, awkward_transform) are now defined as None in the except ImportError branches, so star-imports no longer raise AttributeError when sympy or awkward is absent. __dir__() is also updated to filter these names out when the corresponding dependency is missing.
• Fix Vector.like() to raise TypeError for non-vector arguments (src/vector/_methods.py): The else branch now only triggers for Vector4D; anything else raises TypeError (matching the style of dim()).
• Docstring corrections (src/vector/_methods.py):
  • eta property: removed copy-paste from theta ("in radians, always between 0 and π"); replaced with correct pseudorapidity description including the formula η = −ln tan(θ/2).
  • rotate_euler: Tait-Bryan list was a duplicate of the proper Euler list; corrected to "xzy", "xyz", "yxz", "yzx", "zyx", "zxy" (verified against _compute/spatial/rotate_euler.py).
  • MomentumProtocolLorentz.E: fixed typo "synonyor" → "synonym for".
  • MomentumProtocolLorentz.E2: fixed broken cross-ref :attr:VectorProtocolLorent2 → `:attr:`VectorProtocolLorentz.t2.
  • to_pxpythetamass: corrected "$energy$" → "$mass$" in first-line description.
  • to_ptphietamass: corrected "$\theta$" → "$\eta$" in first-line description.
  • Vector2D.to_Vector4D and Vector3D.to_Vector4D error messages: "At most one longitudinal coordinate" → "temporal" for the temporal over-specification check.

Test plan

• test_star_import_without_optional_deps: subprocess test that blocks sympy via a find_spec meta-path finder, then verifies from vector import * succeeds and sympy names are absent from dir(vector)
• test_like_non_vector_raises: verifies v.like(5) raises TypeError
• test_to_vector4d_temporal_error_message: verifies the error message says "temporal" for both Vector2D.to_Vector4D and Vector3D.to_Vector4D
• 814 existing tests pass, 3 skipped (dask-awkward, optree, jax — all pre-existing)
• All docstring doctests pass (--doctest-plus src/vector/; only pre-existing _pytree.py optree failure)
• prek -a --quiet passes cleanly

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.23%. Comparing base (d9ef1db) to head (4b97068).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #716      +/-   ##
==========================================
+ Coverage   87.16%   87.23%   +0.06%     
==========================================
  Files          96       96              
  Lines       11199    11200       +1     
==========================================
+ Hits         9762     9770       +8     
+ Misses       1437     1430       -7     

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