feat(contract): Oracle freshness, multi-oracle medianization, VRF commit-reveal, keeper bonding (#1040-#1043)

[devdianax]

Summary

Implements four contract-level security improvements as specified in Issues #1040-#1043.

Issue #1040 - Oracle Heartbeat Freshness Verification & Staleness Circuit Breaker

• Added MAX_ORACLE_DELAY_SECONDS = 300 (5 minutes) staleness threshold.
• Implemented validate_oracle_freshness() helper that rejects oracle responses older than the threshold.
• Integrated staleness check into execute_internal pipeline - stale oracle feeds cause execution to revert with Error::OracleStale.

Issue #1041 - Multi-Source Oracle Medianization & Flash Deviation Thresholds

• Added OracleFeed struct for per-provider price feeds (Chainlink, Band).
• Implemented compute_median_price() using insertion-sort median across all fresh feeds.
• Implemented check_oracle_deviation() that panics with Error::OracleDeviationExceeded if any two feeds deviate by more than MAX_ORACLE_DEVIATION_BPS (2.5%).
• Added public methods: set_oracle_feed(), get_oracle_feed(), get_medianized_oracle_price().
• Oracle freshness and deviation checks run automatically during execute_internal.

Issue #1042 - Cryptographic VRF Commit-Reveal Scheme & Fulfillment Window Enforcement

• Added VrfCommit type for the commit phase and commit_vrf_seed() method.
• Added configurable timing via set_vrf_timing_config(delay_seconds, expiration_seconds).
• fulfill_vrf_request() now enforces the fulfillment window - rejects early (VrfFulfillmentTooEarly) or expired (VrfFulfillmentExpired) fulfillments when delay > 0.
• Added cancel_vrf_request() for requestors to reclaim after expiration.
• Defaults to delay=0 (backward-compatible) - admin must explicitly enable via set_vrf_timing_config().

Issue #1043 - Keeper Economic Bonding & On-Chain Stake Slashing Protocol

• Added KeeperBond type and MIN_KEEPER_STAKE = 500 token units requirement.
• Implemented bond_keeper_stake() and unbond_keeper_stake() for keepers to lock/unlock collateral.
• Enhanced slash_keeper() with 50/50 split: 50% to protocol treasury, 50% to whistleblower.
• Slashed keepers are marked is_slashed and locked from participating until rebonded.
• Added set_task_requires_bond(task_id, true) for per-task opt-in.
• Bonding check runs in execute_internal only for tasks marked as restricted.

Acceptance Criteria

• Keepers with balance < MIN_KEEPER_STAKE cannot claim restricted tasks (🦀 [CONTRACT] Keeper Economic Bonding & On-Chain Stake Slashing Protocol #1043)
• Valid fraud proof slashes stake and awards bounty accurately (🦀 [CONTRACT] Keeper Economic Bonding & On-Chain Stake Slashing Protocol #1043)
• Price feed reads older than 300 seconds revert with Error::OracleStale (🦀 [CONTRACT] Oracle Heartbeat Freshness Verification & Staleness Circuit Breaker #1040)
• Deviation exceeding 2.5% halts task execution safely (🦀 [CONTRACT] Multi-Source Oracle Medianization & Flash Deviation Thresholds #1041)
• Fulfillments before window or after expiration are rejected (🦀 [CONTRACT] Cryptographic VRF Commit-Reveal Scheme & Fulfillment Window Enforcement #1042)
• Medianization logic verified across 1, 2, and 3 active feeds (🦀 [CONTRACT] Multi-Source Oracle Medianization & Flash Deviation Thresholds #1041)

Closes #1040
Closes #1041
Closes #1042
Closes #1043

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