Add dry-run execution simulation for governance proposals (#917)

[Kenisank]

Closes #917

Summary

Add a simulation interface that runs proposal logic without mutating state, so maintainers can validate proposal effects before executing on-chain.

Changes

1. proposals.rs

   • Define SimulationEffect and SimulationResult structs (contract types) that describe every storage mutation a proposal would cause.
   • Add simulate_proposal_action(env, proposal) — iterates over all ProposalType variants, reads current storage values, and records the proposed changes in a SimulationResult without writing anything.
   • Add simulate_proposal(env, proposal_id) — convenience wrapper that fetches the proposal by ID and delegates to simulate_proposal_action.

2. lib.rs

   • Import simulate_proposal, SimulationEffect, SimulationResult from the proposals module.
   • Publicly export SimulationEffect, SimulationResult.
   • Add simulate_proposal as a public contract function that calls through to proposals::simulate_proposal.
   • Register est_simulation test module.

3. test_simulation.rs (new)

   • simulate_signal_proposal_returns_effects_without_mutating — verifies simulation returns effects and does not change proposal status.
   • simulate_parameter_change_proposal_reports_current_and_proposed — checks that current/proposed values are surfaced for param changes.
   • simulate_treasury_spend_insufficient_balance_reports_failure — confirms simulation reports failure when treasury balance is too low.
   • simulate_nonexistent_proposal_returns_error — validates that ProposalNotFound is returned for invalid IDs.
   • simulate_feature_toggle_reports_effect — checks feature toggle effects contain the correct key.
   • simulation_is_read_only_no_storage_writes — runs simulation twice and confirms identical results + unchanged state.

Design notes

• The simulation is read-only — no storage writes are performed. Only env.storage().instance().get() is called; no .set() or .remove().
• No authentication required — safe to call via simulateTransaction RPC.
• Simulation failure (e.g. insufficient treasury) is returned as a SimulationResult with success: false rather than a contract error, so the caller always receives structured diagnostic information.

Summary

Test plan

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WASM ABI diff

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