Stop Vault Interest from Creating Unbacked or Repeated Yield

[ahmadogo]

The task has been completed according to all acceptance criteria and constraints.

Summary of Accomplishments
Per-Vault Last-Accrual Timestamp Tracking:

Added VaultKey::LastAccrual(VaultId) in

state.rs
, initialized to created_at on vault creation.
Accrual now calculates elapsed time strictly as elapsed = now.saturating_sub(last_accrual) and advances last_accrual to now.
Eliminated Overlapping & Repeated Accrual:

Implemented in

interest.rs
.
Invocations at the same timestamp yield 0 additional interest (elapsed == 0).
Split period accruals $[t_0, t_1]$ and $[t_1, t_2]$ are strictly disjoint and match $[t_0, t_2]$.
Backed Withdrawable Balances & Informational Yield:

VaultKey::Balance and balances_map strictly reflect deposited principal (1:1 backed by real tokens in vault custody).
Accrued interest is stored in VaultKey::VaultInterest as non-withdrawable informational metrics unless backed by on-chain funding.

Closes #118