Emit structured events from shadow mode dry-run in governance contract

[Mathews-25]

Summary

shadow_mode.rs in contracts/governance/ simulates governance proposal execution without committing state, which is a valuable safety feature. However, the simulation results are only available as return values from a direct contract call. Off-chain monitoring tools and indexers cannot observe shadow-mode execution paths because no events are emitted during a dry run.

Motivation

• Indexers and governance dashboards need a durable record of shadow-mode simulation outcomes to build "what-if" analytics for DAO proposals.
• Emitting events during dry-run (even if the ledger state is not modified) allows event subscribers to detect proposals that would fail on execution before the vote closes.
• This is consistent with the audit trail philosophy in audit_log.rs.

Proposed Implementation

1. Add a ShadowModeResult event type:

   #[contracttype]
   pub struct ShadowModeResult {
       pub proposal_id: u32,
       pub success: bool,
       pub simulated_state_changes: Vec<String>, // human-readable summary
       pub failure_reason: Option<String>,
   }

2. At the end of simulate_proposal in shadow_mode.rs, emit this event regardless of success/failure outcome.
3. Ensure the simulation does not actually write any state (use a read-only check pattern — read storage, compute result, emit event, return result without writing).

Acceptance Criteria / Definition of Done

• ShadowModeResult event is defined in events.rs (or equivalent).
• simulate_proposal emits the event with correct success flag and failure_reason when applicable.
• No persistent storage is written during a shadow-mode invocation.
• Unit tests verify: (a) successful simulation emits event with success = true, (b) failing simulation emits event with success = false and non-null failure_reason.
• cargo test passes with no regressions.

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