dAppStaking truly `stake-and-forget` via infinite Build&Earn

[ipapandinas]

Context

After Tokenomics 3.0 removes bonus mechanics in #1586 and voting periods become ceremonial (1 era), the protocol could become fully permissionless and stake-and-forget by extending Build&Earn indefinitely.

Current Limitation

Setting eras_per_build_and_earn_subperiod = EraNumber::MAX breaks two critical maintenance mechanisms:

• Yearly inflation recalculation (coupled to cycle boundaries via recalculation_era here)
• Storage cleanup (coupled to Build&Earn→Voting transitions via update_cleanup_marker())

Both become inert without periodic cycle transitions.

Risk

• Cleanup logic currently depends on PeriodEnd storage to find oldest_valid_era. Without period transitions, PeriodEnd is never written, so it needs an alternative anchor.
• Both mechanisms are well-tested in their current form; decoupling means new invariants to validate and tests to rework.

This needs investigation to find a proper solution.

Extra Cleanup

• Remove DynamicPercentage
• Remove slot_number_args
• Remove MaxNumberOfContractsLegacy and manually claim all past period rewards.
• Prune old PeriodEnd entries.