feat: Add DAO-compatible SlashingModule with trust decay

[dominusaxis]

Summary

Implements the SlashingModule as requested in #1.

Features

Slash by DAO Vote:

• proposeSlash(target, amount, reason) — any staker can propose
• voteSlash(proposalId, support) — stakers vote for/against
• executeSlash(proposalId) — executes after quorum reached + majority
• Configurable quorum (default: 3 votes)

Slash by Dispute Outcome:

• slashByDispute(target, amount, reason) — owner/DAO direct slash
• Integrates with Escrow dispute resolution

Trust Decay:

• calculateDecay(target) — computes decay based on time since staking
• applyDecay(target) — applies decay (callable by anyone)
• Configurable decay rate (default: 0.5%/day in basis points)

Integration with StakingModule:

• Reads stake data via IStakingModule interface
• effectiveStake(target) — returns stake minus total slashed
• Emits Slashed(address, amount, reason) as specified

Contract: 232 lines

• OpenZeppelin Ownable for DAO admin
• Full proposal lifecycle: propose → vote → execute/cancel
• Slash history tracking per address
• View functions for proposals and effective stakes

Closes #1