feat: fair-LP valuation and MIN_OUT slippage on all swap paths

[sotoJ24]

• issue related: feat: fair-LP valuation and MIN_OUT slippage on all swap paths #49

Summary

Replaces the spot-price LP valuation in adapter-soroswap with a
manipulation-resistant constant-product fair-value formula, and adds
configurable slippage enforcement to every swap and liquidity call.

Closes #49.

Problem

Two vulnerabilities in the previous implementation:

1. NAV manipulation — position_value priced LP positions using the
   AMM spot price (reserve_a / reserve_b). An attacker could sandwich a
   vault deposit or withdrawal (skew reserves → transact at distorted NAV
   → swap back) to extract value from other depositors.

2. Zero slippage protection — every swap_exact_tokens_for_tokens,
   add_liquidity, and remove_liquidity call used MIN_OUT = 0,
   meaning the vault accepted any output no matter how bad.

Changes

common/types.rs

• Added SlippageConfig struct with max_slippage_bps and
  max_price_impact_bps fields (units: basis points, 1 bps = 0.01%)
• Added SlippageConfig::default() (50 bps slippage, 100 bps price
  impact) and min_out(expected) helper for computing floor amounts

common/storage.rs

• Added DataKey::SlippageCfg — a second instance-storage key separate
  from DataKey::Storage so the config can be updated without
  reinitialising the adapter
• load_slippage falls back to SlippageConfig::default() if never set,
  so existing deployed adapters are safe on upgrade

common/error.rs

• Added PriceImpactTooHigh = 9 — operation rejected before hitting AMM
• Added SlippageTooHigh = 10 — swap output fell below computed floor
• All existing error codes preserved

soroswap_pool/mod.rs — core logic change

get_fair_value (replaces spot-price position_value)

Uses the constant-product fair-value formula: