feat(Karma): add minimum karma tier requirement for slashing

[0x-r4bbit]

Introduce a configurable tier gate on Karma.slash() so that only slashers holding a sufficient karma tier can execute a slash.

• Add IKarmaTiers interface and wire karmaTiers + slashTierRequirement state variables into Karma (packed into one storage slot; gap reduced from 30 to 29)
• Add setKarmaTiers() and setSlashTierRequirement() admin setters with corresponding events
• Add internal _checkSlashTierRequirement() — skipped when karmaTiers is address(0); applies uniformly to all callers including admins
• Add slash(account, rewardRecipient, slasher) overload for intermediary contracts (e.g. RLN) that act on behalf of a human initiator; the original two-argument signature is unchanged
• Update RLN.slashReveal() to use the new overload, passing msg.sender so the tier check runs against the human who committed the slash
• Add tests covering tier enforcement, admin parity, zero-address bypass, and event emission
• Wire karma.setKarmaTiers() into DeployProtocol and add a deployment test asserting the link is established
• Update docs (karma.md, admin-guide.md, rln.md)