feat(contract): implement trigger_payout liveness check and claimable…

[Faromzy]

feat(contract): implement trigger_payout liveness check and claimable state
transition

Summary

Rewrites trigger_payout in the Soroban inheritance contract to own the full
payout lifecycle internally, eliminating the need for callers to manually call
claim() before triggering a payout.

Changes

contracts/inheritance-contract/src/lib.rs

trigger_payout now executes three sequential steps in a single call:

1. Liveness check — verifies current_ledger_timestamp > last_ping +
   grace_period. Returns Error::InactivityPeriodNotMet if the owner is still
   considered alive.
2. Claimable transition — on the first call past the deadline, sets
   plan.is_active = false, writes ClaimStatus(owner) = current_time to persistent
   storage, and emits a Claimable event. On repeat calls the original claim_time
   is reused so the timelock window is never extended. When the timelock has not
   yet elapsed, the function returns Ok(()) (not an error) so that the state
   changes are committed on-chain — returning an error would cause Soroban to
   roll back all storage writes, losing the transition.
3. Timelock + distribution — once claim_time + timelock_duration has elapsed,
   tokens are distributed pro-rata to all beneficiaries using their
   allocation_bps. Integer-division dust is absorbed by the last beneficiary so
   the contract never retains stranded tokens.

contracts/inheritance-contract/src/test.rs

• Added 6 new tests covering: timer not met, timer at exact deadline (strictly
  greater-than required), two-call flow (marks claimable → pays after timelock),
  zero-timelock single-call flow, repeated calls do not extend timelock, and
  plan-not-found.
• Updated 4 existing tests whose assertions were tied to the old
  PayoutNotTriggered / TimelockNotExpired error paths that no longer apply.

Call pattern

// First call — grace period elapsed, timelock not yet done.
// Plan is marked claimable (is_active = false, ClaimStatus written). Returns
Ok.
contract.trigger_payout(owner);

// … wait timelock_duration seconds …

// Second call — timelock elapsed, full payout executed.
contract.trigger_payout(owner);

If timelock_duration is zero, the plan transitions to claimable and pays out
in the same call.

Test results

test result: ok. 134 passed; 0 failed (unit tests)
test result: ok. 5 passed; 0 failed (property tests)

closes #918

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[ONEONUORA]

@Faromzy
Pls run cargo fmt, clippy and test