Stealth-sender protocol fee mechanism (opt-in, configurable)

[truthixify]

Tier: L (1-2 weeks) Type: feature

Context

Hosted Wraith providers will need a way to charge a small protocol fee per stealth payment to sustain infrastructure costs. Add an optional fee mechanism to stealth-sender that diverts a configurable percentage to a designated address, off by default for permissionless use.

Scope

• Add fee_recipient: Option<Address> and fee_basis_points: u32 to stealth-sender::init
• When set, on each send/batch_send, divert (amount * fee_basis_points / 10000) to the recipient
• Maximum fee capped at 50 bps (0.5%) by contract invariant
• Fee is sent atomically with the user transfer + announcement (all-or-nothing)
• Off when both fields are zero (current default behavior)

Acceptance criteria

• Contract changes with full test coverage
• Adversarial tests: fee > cap, fee = 0, fee_recipient invalid
• Update SAC compatibility tests to account for fee transfer
• Documentation + governance doc updated

Files to start with

• stellar/stealth-sender/src/lib.rs
• stellar/audits/2026-06-sac-compatibility.md