feat(contracts): fee rounding, pause reasons, dual-approval emergency, storage registry

[miss-yusrah]

closes #639
closes #643
closes #647
closes #648

Summary

Implements four vault contract improvements for operator safety and auditability.

Task 1 — Deterministic fee rounding policy tests

• Added fee_math.rs with floor-division fee policy (fee = amount * bps / 10_000)
• Edge-case tests: zero amount/bps, max bps, sub-stroop truncation, boundary values, monotonicity
• accrue_yield now uses calculate_protocol_fee() for consistent rounding

Task 2 — Paused-state reason codes + query endpoint

• Added PauseReason enum (SecurityIncident, OracleFailure, LiquidityCrisis, Governance, Maintenance, Other)
• pause(reason) stores reason on-chain; unpause() clears it
• New read method: pause_reason() -> Option<PauseReason>

Task 3 — Dual-approval for critical emergency actions

• Added emergency.rs with two-approver flow
• set_emergency_approvers(primary, secondary) — admin configures distinct approvers
• propose_emergency_action(...) — primary initiates (Pause, Unpause, EmergencyDivest, ForceUpgrade)
• confirm_emergency_action(proposal_id) — secondary confirms and executes
• emergency_proposal(proposal_id) — query pending/executed proposals

Task 4 — Storage key namespace registry + collision checks

• Added storage_registry.rs with StorageNamespace and key catalog
• validate_registry_no_collisions() enforces unique (namespace, name) pairs
• New query: storage_key_registry() returns full registry + validity flag
• Proxy ProxyDataKey kept separate from vault DataKey

Test plan

• fee_math edge-case unit tests
• feature_tests — pause reason, dual-approval pause, fee rounding integration, registry validity
• storage_registry collision tests
• emergency distinct-approver tests
• Updated event_tests::test_pause_unpause_works for new pause(reason) signature

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