feat: governance vote salt + quorum decay (#712)

[almuslavish]

Summary

Closes #712. Implements commit-reveal vote salt (prevents precomputation / front-running) and quorum decay (effective quorum decreases linearly toward a configurable floor after a proposal's deadline) for the predictify-hybrid governance module.

Changes

src/governance.rs

• Added Bytes / BytesN imports.
• New StorageKey::VoteCommitment(Symbol, Address) — stores per-voter commitments.
• New GovernanceError variants: NoCommitment, InvalidReveal, CommitmentExists.
• commit_vote(env, voter, proposal_id, commitment) — stores sha256(salt ++ support_byte) during the open voting window; guards against duplicate commits and mixing with direct votes.
• reveal_vote(env, voter, proposal_id, salt, support) — recomputes the hash, verifies against stored commitment, tallies vote with delegation weight, removes commitment to prevent double-reveal.
• Quorum decay (compute_effective_quorum, set_quorum_decay, get_quorum_decay) was already implemented; no changes required.

src/events.rs

• New GovernanceVoteCommittedEvent struct (#[contracttype]).
• New EventEmitter::emit_governance_vote_committed — publishes under topic gov_cmit.

src/governance_tests.rs

• Added Bytes / BytesN imports.
• Added commit_vote, reveal_vote, and make_commitment helpers to GovernanceFixture.
• 8 new focused tests:
  • vote_salt_commit_reveal_full_lifecycle
  • vote_salt_wrong_reveal_rejected
  • vote_salt_duplicate_commit_rejected
  • vote_salt_reveal_without_commit_rejected
  • vote_salt_commit_rejected_after_voting_ends
  • vote_salt_reveal_rejected_after_voting_ends
  • vote_salt_double_reveal_rejected
  • vote_salt_direct_vote_then_commit_rejected
  • vote_salt_commit_not_found_wrong_proposal

Docs

• docs/contracts/GOVERNANCE.md — full rewrite: vote salt API, quorum decay formula/config table, delegation rules, event table, complete error reference, regression coverage index.
• contracts/predictify-hybrid/README.md — added Governance section with quick-reference snippets; replaced placeholder future-enhancement entry.

Security Properties

Property	Mechanism
Precomputation / front-running prevention	Commit-reveal: voters publish sha256(salt ++ support_byte); direction hidden until reveal
Double-vote	Commitment removed post-reveal; Vote key guards both direct and commit-reveal paths
Griefing (re-commit)	CommitmentExists error prevents replacement after commitment
Quorum decay overflow	saturating_mul / saturating_sub throughout; floor enforced
require_auth	Every state-changing entrypoint calls voter.require_auth() or caller.require_auth()

Acceptance Criteria

• Implementation matches description (vote salt + quorum decay)
• Tests added and passing (cargo test -p predictify-hybrid)
• Docs updated
• require_auth on every state-changing entrypoint
• Overflow-safe math (no unwrap() in production paths)
• Clear /// rustdoc on all public items

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