feat(contracts): rounding direction protection for fractional asset math

[godwinishaku506-hub]

Summary

Implements on-chain rounding direction protection for secure and deterministic fractional asset management in the Soroban smart contract, as required by the Wave 4 grant specification.

New module: src/rounding.rs

• RoundingDirection enum — Floor / Ceiling / Truncate variants. Every division that touches token amounts now carries an auditable, grep-able rounding intent at the call site.
• safe_div(env, n, d, direction) — checked integer division handling all four sign combinations correctly; panics with PrinceError::RoundingOverflow on zero denominator or arithmetic overflow.
• safe_mul_div(env, a, b, d, direction) — fused multiply-then-divide over i128 checked arithmetic, preventing overflow at Stellar-scale pool sizes (~5×10¹⁷ stroops).
• dust_remainder(env, pool, distributed) — explicit helper so undistributed dust is carried forward rather than silently lost.

11 proptest invariants (2000 cases each)

#	Invariant
1	Floor never exceeds exact value (floor * d ≤ n)
2	Ceiling never undercounts (ceil * d ≥ n)
3	Floor ≤ Truncate ≤ Ceiling for positive inputs
4	Exact division — all three modes agree
5	Ceiling − Floor gap is at most 1
6	safe_mul_div Floor never over-allocates
7	Sum of N floored allocations never exceeds pool
8	Monotonicity in numerator
9	No panic for any non-zero denominator (adversarial i128::ANY inputs)
10	Commutativity of a × b / d
11	Dust is always < number of projects

Integration

QF distribution (qf_preview_distribution): replaced raw pool * weight / total_weight integer division with safe_mul_div(..., RoundingDirection::Floor). Undistributed dust is now explicitly tracked via dust_remainder and carried forward to the next round in qf_distribute.

Pre-existing bug fixes (required for compilation)

The repository had a broken Cargo.lock (git merge conflict markers) that prevented any build. Fixed alongside the following compile errors:

• Removed 4 duplicate PrinceError discriminants (values 41/42/43 appeared twice each)
• Removed duplicate DataKey::Vault enum variant
• Fixed unclosed delimiter in flash_loan event publish call
• Fixed get_token(&env) → get_token(env.clone()) type errors in create_vault / claim_vault
• Replaced undefined get_org_admin_requirement call in register_bls_signer
• bls_verifier.rs: updated to bls12_381_plus 0.8.x API
• token_interface.rs: sac_burn switched from StellarAssetClient to TokenClient
• Cargo.lock: resolved merge conflict markers; regenerated clean lockfile

Verification

cargo check → 0 errors, 4 pre-existing dead-code warnings (unrelated files)

Note: the test runner has a pre-existing transitive rand_core version conflict in soroban-env-host v22.1.3 that prevents cargo test from building; this exists in the repo independently of this PR.

closes #511