Review historical accuracy of Australia holiday start dates

[coderabbitai[bot]]

This issue tracks the need to verify and potentially correct the historical start dates for certain Australia holidays that may not have been observed as public holidays from the beginning of the country's recorded history.

Holidays to investigate:

1. Labour Day observance - Verify when Labour Day began being observed as an official day off at:

   • Federal level
   • Each state/territory level (particularly for NT "May Day" and TAS "Eight Hours Day" variants)

2. Picnic Day in Northern Territory - Verify the actual start year for this holiday

3. Royal Queensland Show in Queensland - Verify when this became an official public holiday

Context: Currently the Australia tests assert these holidays across the full year range without subdivision-specific start years, but they may not have been official public holidays from the beginning of Australia's recorded holiday history.

Related PR: #2881 Requested by: @PPsyrius in https://github.com/vacanza/holidays/pull/2881#discussion_r2319084662

Priority: Low - historical accuracy improvement, not blocking current functionality