feat(contracts): implement fractional asset wrapping module

[godwinishaku506-hub]

Add packages/contracts/src/fractional_wrap.rs — a self-contained, #![no_std]-compatible module that provides the pure-math kernel for deterministic SAC token wrapping/unwrapping on Stellar Soroban.

Key additions
─────────────
• compute_wrap() — validated deposit → shares conversion (rounds DOWN,
favors pool; rejects zero-share deposits).
• compute_unwrap() — validated shares → asset conversion (rounds UP,
favors pool; rejects excess-share burns).
• preview_wrap() — pure, Env-free preview returning Option.
• preview_unwrap() — pure, Env-free preview returning Option.
• proportional_share() — floor-rounded proportional allocation helper
using safe_mul_div from rounding.rs.
• WrapResult / UnwrapResult domain types carrying all state-update values
so contract dispatch code never re-derives them.

Security properties
───────────────────
• ERC-4626-style +1 virtual share/asset offset defeats inflation attacks. • Rounding direction is explicit and auditable at every call site. • WRAP_MAX_AMOUNT constant prevents i128 overflow in intermediate products. • Zero-share guard blocks donation-vector share-price manipulation. • All arithmetic uses checked_add / checked_mul — no silent overflow.

Proptest invariant suite (11 property tests, 2 000 cases each) ──────────────────────────────────────────────────────────────

1. prop_no_inflation_attack — wrap → immediate unwrap ≤ deposit
2. prop_share_fair_value — shares × (ta+1) ≤ assets × (ts+1)
3. prop_pool_ceiling_on_unwrap — returned ≤ total_assets
4. prop_pool_monotone_after_unwrap — pool never grows on unwrap
5. prop_wrap_monotone — larger deposit → ≥ shares
6. prop_unwrap_monotone — more shares → ≥ assets
   7a. prop_wrap_deterministic — same input → same shares
   7b. prop_unwrap_deterministic — same input → same assets
7. prop_preview_wrap_truncation_invariant
8. prop_two_holder_allocation_bounded
9. prop_zero_share_deposit_math_consistency
10. prop_round_trip_never_profits_large — extended large-scale variant

Module wiring
─────────────
• pub mod fractional_wrap; added to lib.rs.
• No changes to existing contract entry points — additive only.

Build
─────
• cargo check passes cleanly with rust 1.88.0 (4 pre-existing warnings
unrelated to this change).
• cargo test blocked by a pre-existing soroban-env-host rand_core version
conflict that exists on main before this commit.

Closes #[fractional-asset-wrapping]
Execution limit: O(1) arithmetic well under the 30 M instruction cap.

closes #519