Add mixed atom/orbital projection API, kline styling and layered scatter plotting for bands

[LRH123LRH123]

Motivation

• Provide a flexible API to select and plot mixed atom and orbital projections (dict or list styles) and to export their numeric data.
• Allow user control over k-point vertical line appearance and introduce a layered scatter plotting mode to improve visual compositing of projection channels.
• Simplify specifying orbitals by aliases and support f-orbitals when available.

Description

• Implemented orbital name mapping and parsing helpers in Band: _orbital_name_to_index, _get_spd_masks, _parse_orbital_selector, _parse_atom_selector, _normalize_mixed_projection_spec, and _format_orbital_selector_label to convert human-friendly selectors into boolean masks.
• Added mixed-projection aggregation and export features: _sum_mixed_projections, export_mixed_projections_data, and public plotting method plot_mixed_projections that accept projection_spec in both dict and list styles and produce labeled projections and an optional PBAND_mixed.dat export.
• Extended plotting internals: added kline_style and kline_width parameters to _get_kticks, _get_kticks_hse, _get_kticks_unfold, plot_plain, and callers to control vertical k-line linestyle/width, and added kline_style/kline_width to the public band_plain function in standard.py.
• Added a new plotting backend _plot_projected_general_new which supports scatter_mode='flattened' (original behavior) and 'layered' (per-channel draw order) and exposed it via plot_mixed_projections; also introduced scatter_mode and color handling in the band_mixed_projections helper in standard.py.
• Minor API tweaks and defaults: adjusted a few default linewidth values and ensured unfolding logic integrates with the new projection/export code.

Testing

• No automated tests were run for this change.

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Codex Task