feat: switch accrual to e18 fixed-point for tighter rounding

[ayo-ola0710]

What was done

In this implementation, we replaced the previous per-period BPS-based accrual method with an e18 fixed-point accrual index to eliminate cumulative rounding dust. We added an E18 constant for high-precision arithmetic, introduced new storage keys (AccrualIndexE18 for per-offering index tracking and LastClaimedAccrualIndex for per-holder last-claimed index), updated do_deposit_revenue to calculate and update the accrual index on each revenue deposit, modified set_holder_share_internal to set a holder’s last-claimed index to the current accrual index (ensuring they only earn on future deposits), and rewrote the claim function to use the accrual index to calculate total payouts instead of per-period calculations, all while maintaining existing invariants and using checked_mul/checked_div for secure arithmetic.

Close #479

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