units: build PhysicalQuantity with a factory as a frozen result, and split quantity.py into modules

[m-messer]

Stacked on #3: merge that first, and this PR's base will then switch to main.

PhysicalQuantity becomes an immutable result built by a factory, and the code that reads a quantity is split out of quantity.py. Parsing results are unchanged.

Changes

• A factory and a frozen result.
  • PhysicalQuantity.from_tree(name, params, ast_root, parser) runs each step as a separate function: split value from unit, restore the value's text, LaTeX, then the SymPy forms in SI base units. parse_quantity calls it.
  • PhysicalQuantity is a @dataclass(frozen=True, eq=False) with the same attributes and repr. Quantities still compare by identity.
  • The rotations return the new root instead of setting self.ast_root.
  • The quantity no longer holds its parser, so the custom __getstate__/__setstate__ are gone. Pickle and deepcopy just work, and an existing pickle still loads.
• Breaking (CHANGELOG):
  • Setting an attribute raises FrozenInstanceError.
  • reverted_units is a tuple.
  • The constructor takes every attribute; use from_tree, or parse_quantity as before.
  • quantity.parser is deprecated (removed in 1.0). It warns and returns the same cached parser as build_quantity_parser(params.unit_sets, params.strictness).
• Modules. quantity.py keeps the public PhysicalQuantity, RevertedUnit, parse_quantity and same_dimension. The steps move to private modules:
  • _split.py: the rotations, split_value_and_unit, the unit checks and MAX_UNIT_EXPONENT.
  • _latex.py: latex_for_unit and latex_for_value.
  • _conversion.py: quantity_parsing_config and sympy_forms.
  • unsplittable_names(unit_sets, all_forms) moves to data.py, so the new modules don't import each other.
  • Public names are unchanged from lambdafeedback.units. Only lambdafeedback.units.quantity.MAX_UNIT_EXPONENT no longer resolves; nothing here or in compareExpressions uses it.

Downstream
compareExpressions only reads value, unit, unit_latex and latex, and loops over reverted_units, so it isn't affected.

Verification

• ruff, format and mypy are clean, and all four test suites pass. New tests cover:
  • each reading step directly;
  • that the quantity is frozen;
  • that every field survives a pickle round trip;
  • the deprecated parser.
• tools/parity/check.py: 0 unexpected changes.
• A dump of every field (tag trees, repr, LaTeX, SymPy forms, reverted_units) over the parity corpus plus extra juxtaposition inputs, in both strict and natural mode, is byte-identical before and after both commits.
• scripts/check_dist.py passes.

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