Fix transposed Lattice when writing non-symmetric cells

[jameskermode]

Frame.cell stores the lattice vectors as columns (cell[:, i] is a_i). Both readers already return that, for both Lattice syntaxes:

• old-style Lattice="a1 a2 a3": the nine numbers list a1, a2, a3 in turn;
• nested Lattice=[[...], [...], [...]]: the matrix as written, which test_new_non_symm_Lattice pins.

Both writers transposed the cell once more before writing. So a read → write → read round trip transposed every non-symmetric cell. Every existing round-trip test used a diagonal or symmetric cell, where a transpose doesn't show.

Fix (Python only; the C code is unchanged):

• C writer (_write_frame_cextxyz): pass frame.cell. Its old-style output is column-major.
• Python writer (_write_frame_pure_python): write Lattice in the old 9-number form (a1 a2 a3, repr precision), as the C writer and ASE do. ASE's own extxyz reader rejects the nested form this writer used to produce.
• ase-extxyz:
  • _atoms_to_frame stores atoms.cell.T, converting ASE's row vectors to columns.
  • ExtXYZTrajectoryWriter no longer transposes. Until now these two errors cancelled each other, but only for the C trajectory writer.
• Frame docstring: now states the convention. The old text called ASE's convention "column-vector".

Tests:

• tests/test_cell_convention.py: a triclinic cell through all three parsers (C with cleri, C first-char dispatch, Python), in both Lattice syntaxes, through both writers, plus the full write/read round-trip matrix.
• python/ase-extxyz/tests/test_io.py:
  • triclinic round trips across both writers and both readers;
  • the trajectory writer;
  • a check of each written file with ASE's own extxyz reader, an independent parser.
• Locally:
  • core: 107 passed, 2 skipped;
  • ase-extxyz test_io, test_ase_cases, test_kv_parsing_key and the old-style-array tests: 28 passed. The full kv-parsing suite is slow.

One mismatch for the maintainer to settle: the README spec section says a 3x3 Lattice has "rows are cell vectors". That contradicts test_new_non_symm_Lattice and the readers. This PR follows the readers and the test, and leaves the README unchanged.

Found via ace-jax, whose extxyz round trip transposed triclinic cells (ACEsuit/ace-jax#36).

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