[Smart Contract] Build Parametric Insurance Contract with Oracle Triggers

[Osifowora]

Summary

Build an automated parametric insurance contract that pays out automatically based on external data from an oracle feed. This is an MVP-critical feature for Web3 Student Lab's curriculum layer.

Changes

contracts/parametric_insurance/src/lib.rs

Complete rewrite with three core capabilities:

1. Premium deposits — buy_policy() transfers tokens from buyer → contract via token::Client, matching the pattern established in lending.rs.

2. Oracle trigger conditions — post_oracle_value() accepts numeric values per trigger key (e.g., temperature, rainfall, asset price). Each policy stores a trigger_value and trigger_above flag; claim() evaluates oracle_value >= trigger_value (above) or oracle_value <= trigger_value (below).

3. Instant payouts — On successful claim, the payout is tracked as a claimable balance. withdraw_claim() transfers tokens from the contract to the buyer's address, completing the payout.

Capital Pool Management

• Underwriters deposit tokens via underwrite() and withdraw excess capital via withdraw_underwriting(), both protected by solvency checks.
• buy_policy() rejects if total_capital + premium < locked_liability + payout.
• solvency_ratio_bps() reports pool health in basis points.

Testing

10 unit tests covering all paths and edge cases:

Test	Scenario
buys_policy_and_claims_on_trigger	Happy path: above-threshold trigger
rejects_claim_when_oracle_not_set	No oracle value posted → error
rejects_claim_when_trigger_not_met_below	Below-threshold policy, value above threshold → error
claims_when_trigger_below	Happy path: below-threshold trigger
rejects_expired_policy	Claim after expiry → error
prevents_double_claim	Second claim → error
rejects_policy_that_would_break_solvency	Insufficient pool → error
underwriter_can_withdraw	Capital withdrawal with balance check
buyer_can_withdraw_claim	Full cycle: deposit → buy → trigger → claim → withdraw
solvency_ratio_works	Ratio before/after deposits

Acceptance Criteria Met

• Oracle trigger events verify payouts instantly without manual claim approvals
• Premium deposits from policyholders via token transfer
• Trigger conditions fetched from trusted oracle feeds
• Instant payouts to insured addresses when conditions are met

Technical Details

• Language: Rust (Soroban SDK v22)
• Token standard: Stellar Asset Contract (SAC) via token::Client
• Oracle model: Single trusted oracle posts numeric values per trigger key
• Error handling: panic_with_error! with typed InsuranceError enum
• Patterns: Follows established codebase conventions from lending.rs, oracle_aggregator.rs, and state_channel.rs

Closes #718

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