feat: partial withdrawal support across all tier vaults [NF-02]

[yusuftomilola]

Summary

Adds amount: i128 parameter to withdraw and early_exit across vault_l3, vault_l6, vault_l12, and vault_router, enabling partial position exits without forcing a full close-and-redeposit cycle.

Behaviour

Condition	Result
amount < balance	Partial — burns proportional shares, reduces Balance/Shares, LockUntil/Checkpoint untouched
amount >= balance	Full — identical end-state to original all-or-nothing withdrawal (storage entries removed)
amount > balance	Rejected with AmountExceedsBalance = 7

Exit fee (early_exit only): applied only to the withdrawn amount, not the full position. Tier rates unchanged: L3 = 0.50%, L6 = 1.25%, L12 = 2.50%.

Share burn formula: shares_to_burn = (user_shares * amount) / balance — proportional burn preserving the remaining position's share ratio.

Files Changed

• contracts/vault_l3/src/lib.rs — withdraw(user, amount), early_exit(user, amount); new AmountExceedsBalance = 7 error
• contracts/vault_l3/src/test.rs — 6 new tests
• contracts/vault_l6/src/lib.rs — same pattern
• contracts/vault_l6/src/test.rs — 3 new tests
• contracts/vault_l12/src/lib.rs — same pattern
• contracts/vault_l12/src/test.rs — 3 new tests
• contracts/vault_router/src/lib.rs — withdraw(user, tier, amount), early_exit(user, tier, amount); forwards amount to tier vault

Tests per vault

• test_partial_withdraw_reduces_position
• test_full_amount_partial_withdraw_equals_full_withdraw
• test_over_withdrawal_rejected (#[should_panic(expected = "AmountExceedsBalance")])
• test_partial_early_exit_fee_on_withdrawn_amount_only
• test_over_early_exit_rejected
• test_withdraw_before_lock_fails (L3 only)

closes #77

[vercel]

@yusuftomilola is attempting to deploy a commit to the wumibals' projects Team on Vercel.

A member of the Team first needs to authorize it.