docs: Add global Vault ledger entry invariants (3.1.10)

[Tapanito]

Summary

• Adds 12 global invariants under § 3.1.10 Invariants of the Single Asset Vault spec, replacing the TBD placeholder.
• Invariants are derived from VaultInvariant.cpp (visitEntry + universal checks in finalize).

Invariants added

1. Vault.AssetsAvailable >= 0
2. Vault.AssetsTotal >= 0
3. Vault.AssetsMaximum >= 0
4. Vault.AssetsAvailable <= Vault.AssetsTotal
5. Vault.LossUnrealized <= (Vault.AssetsTotal - Vault.AssetsAvailable)
6. If OutstandingAmount == 0: AssetsTotal == 0 and AssetsAvailable == 0
7. If AssetsMaximum > 0: AssetsTotal <= AssetsMaximum
8. MPTokenIssuance(Vault.ShareMPTID).Issuer == Vault.Account
9. AccountRoot(Vault.Account) is a pseudo-account with VaultID pointing here
10. Asset, Account, ShareMPTID are immutable
11. LossUnrealized modified only by lending protocol transactions
12. A Vault is modified only by vault tx types; at most one per transaction

Source

src/libxrpl/tx/invariants/VaultInvariant.cpp — universal checks block in finalize().

[tyalymov]

As written, Invariant 11 and Invariant 12 contradict each other. Invariant 12 says a Vault is modified only by vault transaction types, but Invariant 11 says LossUnrealized is modified by lending protocol transactions, and LossUnrealized is a field on the Vault. So a lending transaction that updates it (LoanManage on impair/unimpair/default) violates Invariant 12.

The code actually matches Invariant 11, not Invariant 12. VaultInvariant.cpp (finalize) gates vault modification on the MustModifyVault / MayModifyVault privileges, not on "vault transaction types." Lending transactions hold those privileges too: LoanManage has MayModifyVault, and LoanSet / LoanPay have MustModifyVault. That privilege is what lets LoanManage write Vault.LossUnrealized.

Could we reword Invariant 12 to match, along the lines of: a Vault is modified only by transactions authorized to modify it (the vault transactions, plus lending transactions holding MayModifyVault / MustModifyVault)? That also removes the conflict with Invariant 11.