Replace nil-returning year-gate functions with year_ranges

[ppeble]

Summary

• Converts all functions that returned nil purely as a year-range gate to use the year_ranges definition field instead
• Simplifies or removes the affected custom methods now that the year logic lives in the definition
• Adds boundary tests for every changed year range in both pl.yaml and au.yaml

Changes

pl.yaml

• Removes pl_trzech_kroli and pl_trzech_kroli_informal functions; replaces with mday: 6 + year_ranges: from: 2011 / until: 2010

au.yaml

• march_pub_hol_sa: adds year_ranges: from: 2006; simplifies function
• may_pub_hol_sa: adds year_ranges: until: 2005; simplifies function
• qld_queens_birthday_june: adds year_ranges: until: 2015; simplifies function
• qld_labour_day_may: splits into two definitions (until: 2012, from: 2016); simplifies function
• qld_labour_day_october: adds year_ranges: between: {start: 2013, end: 2015}; simplifies function
• qld_queens_bday_october: splits into mday: 1 + limited: [2012] and function + between: {start: 2016, end: 2022}; simplifies function
• qld_kings_bday_october: already had year_ranges: from: 2023; drops nil branch from function
• g20_day_2014_only: removes function entirely; replaces with mday: 14 + limited: [2014]

What was not changed

Functions where nil is a legitimate computation result (not a year gate) are left as-is: jp_citizens_holiday, jp_substitute_holiday, us_inauguration_day, even_year_election_day.

Test plan

• All existing tests pass
• New boundary tests added for every changed year range

[ppeble]

This is as no-op and simplify codifies existing behavior. For this reason I am not waiting for additional approval.