Optimize Coordinates construction and de-duplicate coordinate syntax docs

[kripnerl]

Coordinates.init parsed the rich input syntax on every construction
(~7 us even for a single point), which dominated single-point evaluations
such as eq.B_R(R, Z) (~half of the 25 us call) and the field-line tracer
RHS, which builds three Coordinates objects per ODE evaluation.

Performance:

• Add Coordinates.from_coords() classmethod as the canonical rich-syntax
  factory; Coordinates.init now takes a fast path for the common case
  of 1-3 positional scalars/1-D arrays in the default coordinate type and
  emits a DeprecationWarning for direct rich-syntax construction
  (named coordinates, (N, dim) arrays, non-default coord_type).
• Cache cocos_coefs() with lru_cache and return a read-only mapping
  (it is a pure function of the COCOS index and was rebuilt per init).
• Move the _valid_coordinates* sets to class attributes.
• Clean up the slow parser (module-level Iterable import, single
  np.atleast_1d conversions, set comparison instead of permutation scans;
  3D named input is now also converted with np.atleast_1d).
• Migrate internal call sites (Equilibrium.coordinates, Surface/
  FluxSurface, metis.py, examples, tests) to the warning-free paths.

Measured: Coordinates(eq, 0.7, 0.1) 7 -> 2.1 us; kwargs construction
7 -> 3.0 us; eq.B_R(0.7, 0.1) 25 -> 20.7 us. Bulk array calls are
spline-bound and unchanged.

Documentation:

• The coordinate input syntax was fully documented only in
  Coordinates.init and duplicated in docs/source/coordinates.rst,
  while the ~44 coordinate-accepting Equilibrium methods documented it
  not at all (empty :param: stubs). The syntax now lives once in
  COORDINATES_DOC/COORD_PARAMS_DOC and a new append_to_doc decorator
  injects the full text into Coordinates.from_coords and
  Equilibrium.coordinates, and a short cross-referencing note into all
  coordinate-accepting Equilibrium methods; the empty :param: stubs are
  removed and coordinates.rst embeds the canonical docstring via
  automethod instead of a copy.

Tests: add fast/slow path parity tests and deprecation tests; full suite
777 passed. Sphinx build warning count drops from 10 to 8 (remaining
ones pre-date this change).

https://claude.ai/code/session_01KvV66UPfmyCtKH71MthmWo