fix(lending_pool): add slippage bounds and virtual-share offset to deposit/redeem

[Akpolo]

Vulnerability

lending_pool priced shares from the pool's live token balance
(total_assets = token::Client::balance(pool) + outstanding), and neither
deposit nor withdraw accepted a caller-supplied price bound.

Both were exploitable within a single ledger:

1. Pool is empty. Attacker calls deposit(attacker, 1) → 1 share (first-depositor path).
2. Attacker transfers a large amount of the asset directly to the pool's
   address via the token contract. This is not deposit, so no shares
   are minted, but the live balance the pool reads for pricing jumps.
3. Victim's next deposit gets rounded down (in the worst case to zero
   shares) because the pool's "total assets" figure was inflated by the
   donation.
4. Attacker redeems their single share and extracts the victim's principal
   plus the donation.

Fix (contract layer)

• Internal accounting instead of live-balance pricing. Added
  DataKey::TotalManagedAssets, mutated only inside deposit,
  redeem/withdraw, and the new distribute_yield accrual path — never
  derived from token::Client::balance. An unsolicited transfer to the
  pool's address is now a complete no-op for pricing.
• Virtual shares/assets offset (ERC4626-style decimals offset, 10^3)
  in calc_shares_to_mint/calc_assets_to_redeem, which also unifies what
  was previously a special-cased first-depositor branch.
• Slippage bounds. deposit takes min_shares_out; withdraw /
  emergency_withdraw take min_assets_out. Settlement reverts with
  PoolError::MinSharesNotMet / MinAssetsNotMet instead of executing at a
  worse price than the caller expected. Added PoolError::ZeroShares for
  settlements that round to zero.
• preview_deposit / preview_redeem read-only views for off-chain
  quoting, matching settlement math exactly (rounding included).
• distribute_yield(from, token, amount) — the sole legitimate path for
  realized yield to raise the share price. It performs the real token
  transfer itself and requires from's authorization, so it cannot be used
  to move the price at someone else's expense.
• PriceUpdated event emitted on every pricing mutation
  (ledger_seq, total_managed_assets, total_shares).

Also removed dead code that had been appended to lib.rs/test.rs by a
previous commit (a duplicate top-level deposit function colliding with
the events module's deposit, and a test referencing undefined helpers)
which left the crate not compiling at all before this change.

Test plan

All in contracts/lending_pool:

• cargo build -p lending_pool (and with overflow-checks=on)
• cargo test -p lending_pool — 66 passed, 0 failed
• cargo clippy -p lending_pool --all-targets -- -D warnings — clean
• cargo fmt -p lending_pool -- --check — clean
• New test test_donation_to_pool_address_does_not_move_share_price
  replays the exact extraction walkthrough from this issue end-to-end
  against the fixed contract and asserts: the donation doesn't move
  total_managed_assets or get_share_price, the victim is minted
  fair (undiluted) shares, and the attacker cannot redeem more than
  their own principal.
• test_virtual_offset_prevents_first_depositor_inflation_attack checks
  the offset math in isolation (a victim's shares can no longer round to
  zero from a manipulated pricing input).
• test_deposit_rejects_when_below_min_shares_out /
  test_withdraw_rejects_when_below_min_assets_out /
  test_deposit_succeeds_when_min_shares_out_is_met cover the new
  bound-rejection and success paths.
• test_distribute_yield_requires_source_authorization and
  test_preview_deposit_and_preview_redeem_match_settlement.
• test_round_trip_deposit_then_redeem_is_never_profitable — a
  fixed-seed pseudo-random sequence of 25 rounds, each preceded by a
  donation to the pool's address, asserting deposit(d) immediately
  followed by redeeming all resulting shares never returns more than
  d (round-trip non-profitability).
• All 9 pre-existing yield/loan-cycle tests updated to use the new
  distribute_yield accrual path in place of a raw donation-style
  mint to the pool's address, with expected values recomputed for the
  virtual-offset formula.

Out of scope

• Backend (poolQuoter, quotes route, eventIndexer price ingestion,
  migration) and frontend (useRedeemQuote, RedeemForm, slippage env
  vars) layers, and the cross-layer e2e/CI wiring, are not part of this PR.
  This PR addresses the contract-layer vulnerability and its acceptance
  criteria; the other layers need their own follow-up work against the
  deposit/withdraw/preview_*/PriceUpdated surface added here.
• The workspace-wide cargo build --manifest-path contracts/Cargo.toml
  currently fails on main for loan_manager and multisig_governance
  due to the same kind of pre-existing dead/orphaned code (unrelated
  functions referencing Self outside an impl block) that this PR
  removes from lending_pool. That is unrelated to this issue and out of
  scope here; loan_manager only calls lending_pool's is_paused/
  pool_balance, so it is unaffected by the signature changes in this PR.
• contracts/fuzz (excluded from the workspace) still calls the old
  3-argument deposit/withdraw; updating the fuzz harness to the new
  signatures is a follow-up.

Closes #1380