Replace FastLane with buffered ether allocation

[Psirex]

This PR replaces FastLane with buffered ether allocation and reservation logic.

Buffered ETH is now handled as prioritized parts: depositsReserve, withdrawalsReserve, and unreserved remainder. The diagram below illustrates buffer partitioning:

   ┌─────────── Total Buffered Ether ───────────┐
   ├────────────────────┬───────────────────────┼─────┬──────────────┐
   │●●●●●●●●●●●●●●●●●●●●│●●●●●●●●●●●●●●●●●●●●●●●●○○○○○│○○○○○○○○○○○○○○│
   ├────────────────────┼───────────────────────┼─────┼──────────────┤
   └─ Deposits Reserve ─┼─ Withdrawals Reserve ─┘     ├─ Unreserved ─┘
                        └───── Unfinalized stETH ─────┘

    ● - covered by Buffered Ether
    ○ - not covered by Buffered Ether

    depositsReserve    = min(total, stored deposits reserve)
    withdrawalsReserve = min(total - depositsReserve, unfinalizedStETH)
    unreserved         = total - depositsReserve - withdrawalsReserve

The reserve target behavior is designed to be safe for report processing: decreasing depositsReserveTarget is applied immediately, while increasing it is applied on report sync, so in-flight withdrawal finalization assumptions are not broken.

Tests were updated to reflect the new behavior and validate the main invariants around deposits reserve, withdrawals reserve, and finalization budget calculation.