Model the documented PALS root node properties

[ax3l]

Summary

First PR of the series working down tests/standard_examples_known_failures.txt: PALSroot now models the root keys documented in the standard's Fundamental Structure chapter.

• authors: list of Author (name required; orcid, affiliation, email optional). The standard's one-key - author: entry form is unwrapped on load and restored on dump.
• notes and reminders: lists of strings.
• phase_space_coordinates: accepted as a string. The standard documents four values (ANGLE_AND_ENERGY, ANGLE_AND_MOMENTUM, KINETIC_AND_ENERGY, KINETIC_AND_MOMENTUM), but its own root_keys example writes standard, so the value is not restricted for now.
• extension_labels: either the registered ExtensionLabels form (names/prefixes/suffixes sections, extra="forbid" so it cannot swallow the other form) or the flat label → description dict used by the standard's loading examples.
• load: list of file names (with the literal SELF). Entries are modeled so the documents read and round-trip; resolving them into a combined document is left for a follow-up PR.
• facility is now optional, since joiner and settings-only files carry none.
• version also accepts bare numbers (version: 1), as written by the standard's root_keys example.

The facility item unpacking moved into a reusable unpack_element_items() helper so PALSroot no longer routes through the one-key-name unwrapping that only applies to named containers; BeamLine/UnionEle/Lattice behavior is unchanged. PALSroot.model_dump() keeps version visible even when unset (as before) and drops unset optional keys.

21 files leave the known-failures list; the remaining entries are facility-level include resolution and the sequence variables / compact sets forms.

Known limitations (follow-up material)

• A facility element dict that fails element validation silently falls back to PlaceholderName in the element union (pre-existing). Combined with Quadrupole's stricter-than-standard "at least one multipole group" check, the standard's root_keys q1 loads as a name reference rather than a Quadrupole. Worth a dedicated PR.
• load and include entries are parsed, not resolved.

Testing

• pytest tests/ — 49 passed.
• tests/validate_standard_examples.py against the pinned standard commit (5c39fde): 65 files checked, 0 unexpected results.
• New tests/test_root.py covers the root keys, version forms, facility-less files, both extension_labels forms, YAML/JSON round-trips, and the serialized document shape.

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