[adebayoahmad78-tech]

…g, contract upgrade, emergency pause

#1011 — emergency_pause.rs

• Named pause conditions with severity levels (Info/Warning/Critical)
• Auto-pause on Critical condition trigger
• Multi-step recovery plan with per-step completion tracking
• Notification subscriber list (external contracts called on pause/unpause)
• Aggregate analytics: pause count, duration, critical trigger count

#1010 — contract_upgrade.rs

• Semantic version management (major.minor.patch)
• UpgradeManifest: breaking changes, new features, migration flag
• Pre-upgrade validation (version increasing, hash match)
• Upgrade history log (bounded, 20 entries)
• Rollback capability: stores previous WASM hash, capped rollback counter

#1009 — audit_logging.rs

• Compliance log with monotonic sequence numbers
• LogEventKind: AdminAction, InvoiceLifecycle, Payment, SecurityEvent, UpgradeEvent, RateLimitEvent
• Data retention policy (configurable window, purge on demand)
• Tamper-evident data_hash field (SHA-256 of off-chain encrypted payload)
• Access-controlled reader list (separate from admin/operator roles)
• Analysis helpers: count_by_kind, entries_in_range, time-range queries

#1008 — rate_limiting.rs

• Per-user per-operation sliding-window counter
• Exponential backoff on repeated violations (configurable steps + cap)
• Admin override: reset a user's state; remove a config
• Aggregate analytics: total checked/allowed/denied, per-op denial counts
• Denial-rate in basis points (0–10 000)

Supporting changes:

• error.rs: add RecoveryIncomplete = 77 with recovery suggestion
• lib.rs: declare pub mod for all 4 modules; expose ~30 new entry points
• .gitignore: add rules for .xdr/.base64 artefacts, .soroban/, editor state
• closes Add rate limiting and throttling #1008
• closes Implement audit logging for compliance #1009
• closes Add contract upgrade mechanism #1010
• closes Implement emergency pause mechanism #1011