radiation_weighting formula is unusable with parse_unit('1') — none vs ratio

[withtwoemms]

The shipped radiation_weighting formula cannot be used with the obvious dimensionless literal.

Reproduction

from ucon import Number, parse_unit
from ucon.system import active_kinds
L = active_kinds()

D  = Number(quantity=2,  unit=parse_unit('Gy'), kind=L.get('absorbed_dose'))
wR = Number(quantity=20, unit=parse_unit('1'),  kind=L.get('radiation_weighting_factor'))

KindDimensionMismatch: Kind 'radiation_weighting_factor' has dimension
Dimension(none), but unit 'fraction' has dimension Dimension(ratio).
A Number's kind must refine the dimension of its unit.

Cause

radiation_weighting_factor is declared dimension = "none" (ucon/comprehensive.ucon.toml:5230), but parse_unit('1') resolves to fraction, whose dimension is ratio. The __post_init__ check at ucon/core/_types.py:1631 then refuses the construction.

Constructing a Dimension.none unit by hand works, and the formula itself is correct:

none_unit = Unit(name='one', dimension=Dimension.none)
H = D * Number(quantity=20, unit=none_unit, kind=L.get('radiation_weighting_factor'))
# → 40, kind: dose_equivalent      ✓ H = D · w_R per ICRP 103

Why it matters

The formula ships in the default registry, so it is presented as ready to use. A caller reaching for it writes parse_unit('1') — the natural spelling for a dimensionless multiplier — and gets a type error naming two dimensions they did not choose. Getting it right requires knowing that none and ratio are distinct and that no shipped unit carries none.

Possible directions

• Declare the kind as ratio so it matches the dimensionless unit callers actually have, if that is semantically acceptable.
• Ship a none-dimensioned unit (or make parse_unit('1') yield one) so there is a spelling that works.
• Or, if the distinction is deliberate, say so in the formula's notes and give the working incantation — the error message explains what is wrong but not what to write instead.

Found while verifying the formula for #303. Unrelated to that issue's disposition.