feat(governance): add event emission coverage and validation tests (#…

[Philzwrist07]

Summary

Closes #482 Fixes a bug where execute() and cancel() emitted the generic proposal_finalised event, making
them indistinguishable from a finalisation on the event stream. Adds dedicated events for every
state transition and validates all event payloads in tests.

Changes

events.rs

• Added proposal_executed (topic: "executed") — emits the admin address
• Added proposal_cancelled (topic: "cancelled") — emits the admin address

lib.rs

• execute() now emits proposal_executed instead of proposal_finalised
• cancel() now emits proposal_cancelled instead of proposal_finalised

test.rs

• test_event_proposal_created — validates "created" topic and proposer payload
• test_event_vote_cast — validates "vote" topic, voter, direction, and weight
• test_event_proposal_finalised — validates "final" topic and Passed status
• test_event_proposal_executed — validates "executed" topic and admin payload
• test_event_proposal_cancelled — validates "cancelled" topic and admin payload

docs/events.md (new)

• Full event schema for all 6 event types (topics + data types + JSON examples)
• Off-chain indexer integration guide using Stellar RPC getEvents
• History reconstruction approach — no storage reads required
• Schema stability policy (event schema is a public API)

Acceptance Criteria

┌───────────┬────────┐
│ Criterion │ Status │
├───────────────────────────────────────────────────────────────────────┼────────┤
│ Proposal creation, voting, finalisation, and cancellation emit events │ ✅ │
├───────────────────────────────────────────────────────────────────────┼────────┤
│ Event contents are validated in tests │ ✅ │
├───────────────────────────────────────────────────────────────────────┼───────────────────┤
│ Off-chain indexing needs are documented │ ✅ docs/events.md │
├───────────────────────────────────────────────────────────────────────┼───────────────────┤
│ No silent state changes without events │ ✅ │
└───────────────────────────────────────────────────────────────────────┴────────────────