[guptakumarranjeet150]

Closes #78

Summary of Changes

Implements a formal, canonical event schema versioning standard across audit-registry, usage-meter, and payment-router before downstream integrators and indexers depend on unversioned topics:

1. Canonical Schema Version Constant:
   • Added pub const EVENT_SCHEMA_VERSION: u32 = 1; across all contracts.
2. Versioned Event Topics:
   • Updated all event definitions (CallerAllowed, CallerRevoked, AttestationRecorded, AttestationSuperseded, UsageRecorded, PolicySet, RegistrySet, EscrowOpened, PartialReleased, Claimed, Disputed, DisputeResolved, EscrowClosed, SettlementFailed, SettlementRecovered, DeadLettered) to include #[topic] pub version: u32 as their first indexed topic.
   • Updated all event publication call sites across the contracts to emit EVENT_SCHEMA_VERSION.
3. Comprehensive Event Catalogue & Compatibility Documentation:
   • Added docs/event-catalogue-and-schema-versioning.md documenting every contract event, topic layout convention, payload mapping, emitting entrypoints, and the backward-compatibility / migration policy.
4. Regression Shape Assertions:
   • Added tests in all contracts (audit_registry::test::test_event_schema_version_and_shape, usage_meter::test::test_usage_meter_event_schema_version_and_shape, payment_router::test::test_payment_router_event_schema_version_and_shape) verifying exact topic indices, schema version presence, and payload shapes.
   • All 69 unit and invariant tests across the workspace pass cleanly.