✨ feat(unitsystems): name the Planck and atomic base units

[nstarman]

Closes #868.

astropy.units has no names for the Planck or Hartree atomic base units, so
they could only be spelled as a scale times an SI unit — and to_string()
truncates past six significant figures, so that spelling neither read well nor
reparsed:

>>> planck["length"]
Unit("1.61626e-35 m")          # before — and does *not* reparse
>>> planck["length"]
Unit("l_P")                    # after

Eight units: l_P, m_P, t_P, T_P for Planck; a_0, m_e, t_au, e
for Hartree atomic.

Scope

Only these two systems get names. hep and geometrized take a free scale
(an energy and a length respectively), so their bases change with the
parameter — verified — and no fixed name would be correct:

>>> unitsystem(HEPUSysFlag)["length"]              # 1.97327e-16 m
>>> unitsystem(HEPUSysFlag, energy="TeV")["length"] # 1.97327e-19 m

Three traps, each checked rather than assumed

• a0 and Eh are not free. astropy already parses them as
  dimensionless and as a time (E×h, hour). Either would have silently
  meant the wrong thing. Hence a_0, and no Hartree-energy unit here.
• A unit named e does not disturb scientific notation — 1e5 m and
  1.5e3 kg still parse as a scale times a unit. Tested, since this was the
  main worry with the conventional name for elementary charge.
• astropy ignores a format string identical to the unit's own name and
  then escapes the underscore, so a_0 renders $\mathrm{a\_0}$ unless the
  override differs — a_{0} fixes it.

Values track astropy

Each unit is defined from the same constant expression that already built
its unit system, not from a hard-coded number, so a CODATA revision in
astropy.constants moves these with it. A test pins that.

That also means doctests which hid these behind ... no longer need to: a
name is stable across CODATA revisions where six significant figures were not.

The registry trade-off

Registered with add_enabled_units at import, so unxt.unit("l_P") resolves
and the spelling round-trips. This mutates astropy's registry process-wide —
it is exactly why the change was kept out of #855, and the trade is
deliberate. All eight names were checked unclaimed.

Display alone would not have needed this (a def_unit knows its own name);
parsing does.

Follow-up: #876 tracks upstreaming these into astropy, which would let unxt
drop the module entirely.

Interaction with #855

#855 should rebase on top of this. Its unit-system repr routes a system whose
units have no short exact spelling to the realization's registered name
(unitsystem('planck')). With these units named, planck's bases do spell
exactly, so it will instead render unitsystem(['l_P', 'm_P', 't_P', 'T_P']) —
informative and round-trippable, and quite possibly making that name-lookup
path redundant.

Verification

nox -s "pytest(package='unxt')" — 4068 passed. Every other package session
green (interop_gala, parametric, api, hypothesis, interop_xarray),
plus unxts.linalg 287. pylint 10.00/10, ruff clean.

[Copilot]

Pull request overview

This PR introduces explicit, parseable names for the base units of the fully-determined natural unit systems (planck and Hartree atomic) so they round-trip through astropy.units string formatting/parsing without losing precision due to to_string()’s significant-figure truncation.

Changes:

• Add a new named_units module defining l_P, m_P, t_P, T_P, a_0, m_e, t_au, and e, and register them via astropy.units.add_enabled_units so they are parseable.
• Switch the PlanckUSysFlag and AtomicUSysFlag realizations to use the named units (and update doctest outputs accordingly).
• Add unit tests covering naming, round-tripping, LaTeX formatting, scientific-notation safety for e, and ensuring astropy’s pre-existing a0 meaning remains unchanged.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/unit/test_unitsystems.py	Adds tests asserting named natural bases, exact reparsing, LaTeX formatting, and safety checks around e/a0.
src/unxt/_src/unitsystems/named_units.py	Defines and registers the eight named units for Planck and Hartree atomic systems.
src/unxt/_src/unitsystems/flags.py	Updates doctest outputs for PlanckUSysFlag / AtomicUSysFlag to show named units.
src/unxt/_src/unitsystems/core.py	Uses PLANCK_UNITS / ATOMIC_UNITS instead of recomputing unnamed scaled SI units.
docs/guides/units_and_systems.md	Updates documentation examples to reflect named base units.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.96%. Comparing base (6888db8) to head (4c03f6f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #875      +/-   ##
==========================================
+ Coverage   99.82%   99.96%   +0.13%     
==========================================
  Files          84       47      -37     
  Lines        3985     2700    -1285     
  Branches      311      162     -149     
==========================================
- Hits         3978     2699    -1279     
+ Misses          3        0       -3     
+ Partials        4        1       -3     

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