Bug: errors in handing physical quantities #285

Description of the bug

Discovered by Claude while auditing the physical quantities features:

Physical quantity mode: errors found in code review

Unit table (app/utility/unit_system_conversions.py)

• Roentgen expands to kelvin/kilogram; should be coulomb/kilogram. Docs table has the same error.
• Angle conventions are mixed. Degree = 1/360 and radian = 1/(2*pi) (one turn = 1), but angleminute = pi/10800 and anglesecond = pi/648000 assume radian = 1. Result: 60 angleminute != 1 degree.
• rad maps to the absorbed-dose unit (0.01 Gy); radian uses r. Input 3.14 rad parses as a dose with dimension m^2/s^2.
• Litre plurals are one string, ('litres,liters',); neither plural is recognised. Should be ('litres', 'liters').
• Comment says short forms of set_of_common_units_in_SI are unused, but SLR_generate_unit_dictionaries loads x[1] for all units. Consequences: da also maps to deci-are; with units_string without imperial, ft parses as femto-metricton.
• Truncated constants: pint 0.00056826 (exact 0.00056826125), electronvolt 1.60218e-19 (exact 1.602176634e-19), atomic mass unit 1.66054e-27 (1.66053906660e-27).
• Bel comment says "log(10) in base 2"; the value is the natural log of 10.
• Docs only: gallon listed as 4546.09 litre (should be 4.54609), barn in metre (should be metre^2), kelvin symbol shown as k, litre symbol shown as l (code uses L).

Transforms (app/utility/physical_quantity_utilities.py)

• transform_prefixes_to_standard: the (?<!\w) lookbehind fails after a digit. 5 μm converts, 5μm does not, and μ is not in the unit dictionaries.

Comparison (app/context/physical_quantity.py)

• is_proportional: the elif repeats the test on expr0; the expr1 branch never runs.
• <, >, <=, >= criteria: with mismatched dimensions, bool() on an undecided SymPy relational raises TypeError. No tolerance is applied to these comparisons.
• feedback_string_generator is a stub returning None.
• Docs say criteria are ignored when physical_quantity is true; the code supports matches, dimension and comparison criteria.

Parser (app/utility/slr_parsing_utilities.py)

• ExprNode uses a mutable default tags=set(), shared by all nodes created without a tag_handler. copy() also passes the same set object to the copy.
• SLR_Parser.parse: output += ExprNode(self.parse(tokens), []) passes a list where a Token is expected; this path would raise.
• parse_quantity rebuilds the SLR parser table on every evaluation. Cache by (units_string, strictness).

Expression(s)/input that trigger it

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Expected vs. actual result

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Steps to reproduce / where it was used

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