feat(contracts): implement virtual yield accrual logic

[Ebenezer199914]

Summary

Fixes #831 — [Contracts] Implement Virtual Yield Accrual Logic

Implements fully on-chain virtual yield accrual for the Soroban inheritance contract. Yield is computed using an APR formula and accrued lazily (at ping time) or projected virtually (at read time) without external vault interactions.

Changes

lib.rs

• Plan struct: Added accrued_yield: i128 field (initialised to 0 on plan creation).
• SECS_PER_YEAR constant: 31_536_000 seconds used for APR → per-second conversion.
• compute_accrued_yield(principal, rate_bps, elapsed_secs): Private helper implementing principal * rate_bps * elapsed / (10_000 * SECS_PER_YEAR) with saturating arithmetic.
• ping(): Accrues yield since last ping into plan.accrued_yield before resetting last_ping.
• trigger_payout(): Finalises pending yield at claim time; distributes principal + accrued_yield across beneficiaries.
• reclaim(): Finalises pending yield and transfers principal + accrued_yield back to the owner.
• get_plan(): Projects pending yield since last ping into the returned struct without writing to storage (pure read).

test.rs

Nine new yield-specific tests:

Test	What it checks
test_yield_not_accrued_when_disabled	earn_yield=false keeps accrued_yield=0
test_yield_accrues_over_one_year	10% APR × 1 year × 1000 principal = 100
test_yield_zero_at_creation	No yield at t=0
test_ping_accrues_yield	Ping persists elapsed yield to storage
test_ping_does_not_accrue_yield_when_disabled	Ping is no-op when disabled
test_multiple_pings_accumulate_yield	Two half-year pings == one full-year
test_trigger_payout_includes_yield	Beneficiary receives principal + yield
test_reclaim_includes_yield	Owner reclaims principal + yield
test_get_plan_projects_yield_without_persisting	get_plan is idempotent / pure

CI

• No new clippy warnings (all existing #[allow] attributes preserved).
• rustfmt-formatted.
• All 24 existing tests continue to pass; 9 new tests added.

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[ONEONUORA]

@Ebenezer199914
Pls fix conflict