[icweaver]

NOT READY FOR REVIEW

The numeric core is now units-free. Unitful and DynamicQuantities support live in package extensions that load with using Unitful / using DynamicQuantities.

Core

• Unitful and DynamicQuantities are now weakdeps; PhysicalConstants and UnitfulAstro are dropped.
• Physical constants are the CODATA2018 / SI-2019 exact values in SI units, defined once in core; both extensions build their unitful constants from
  them.
• Backend hooks (_ustrip, _velocity_ratio, _spectral_axis_factor) have unitless defaults and are specialized per extension. A plain Real
  velocity is still read as km/s, and a unitless axis is still treated as wavelength.
• Unitless blackbody is computed in plain floats (last-digit change in its doctest output).

SpectrumBaseUnitfulExt

• spectrum constructors for Quantity axes (wavelength, frequency, energy, including BinnedSpectrum), ustrip(spec), unit(spec),
  blackbody(::Quantity...).
• Redshift factors chosen by dispatch on the axis type: Length stretches, Frequency/Energy compress.
• uconvert conversions of the spectral axis and flux density, powered by UnitfulEquivalences.

SpectrumBaseDynamicQuantitiesExt

• Same constructors, redshift/doppler_shift and blackbody support, plus ustrip(spec) and dimension(spec) (DynamicQuantities has no unit
  objects).
• DynamicQuantities doesn't encode dimension in the type, so dimension checks and redshift factors inspect the axis value. @inferred redshift/doppler_shift still hold.

Docs and tests

• Documenter pulls docstrings from the extension modules; new "DynamicQuantities helpers" section.
• New test/ext/dynamicquantities.jl; doctests now also cover extension docstrings.

Caveats and follow-ups

• Unitful isn't optional in practice yet: DustExtinction (a hard dependency) loads Unitful, so the Unitful extension is always active. Truly
  optional Unitful needs DustExtinction to become optional too.
• UnitfulEquivalences stays a hard dependency so uconvert works with only using Unitful. The alternative is a separate ["Unitful", "UnitfulEquivalences"] extension, at the cost of requiring using UnitfulEquivalences.
• No uconvert equivalent for DynamicQuantities yet; it needs its own E = hν = hc/λ and F_ν = F_λ λ²/c implementation.

[codecov]

Codecov Report

❌ Patch coverage is 92.40506% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.69%. Comparing base (ed81082) to head (d821e2c).

Files with missing lines	Patch %	Lines
ext/SpectrumBaseDynamicQuantitiesExt.jl	83.33%	5 Missing ⚠️
ext/SpectrumBaseUnitfulExt.jl	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
- Coverage   96.05%   94.69%   -1.36%     
==========================================
  Files          11       12       +1     
  Lines         228      264      +36     
==========================================
+ Hits          219      250      +31     
- Misses          9       14       +5     

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