feat(registry): add name pricing with length-based registration fees #1

💠 Sidera — Stellar Name Service. Contribute via Drips Wave / GrantFox. Comment with your approach and wait for assignment before starting.

Description

Registration is currently free (register charges nothing), which invites name-squatting: one account can register thousands of short valuable names (bob, xlm, usdc) at zero cost. A paid registry needs a length-based fee — shorter names cost more — so scarcity is priced instead of grabbed.

Desired outcome: register requires an XLM payment attached to the invocation, priced by a public length-tier function, with the fee held by the contract (treasury distribution is a separate tranche).

Scope

• Crate/module: crates/registry, src/lib.rs
• API: new fn price_for(env, name) -> i128 read-only view; register gains a fee check on the attached payment
• Storage: new DataKey::FeeConfig (instance) for tier boundaries; contract XLM balance implicitly holds fees
• Tests: pricing tiers (3–4 / 5–7 / 8–31 / 32 chars), insufficient-payment rejection, exact-payment acceptance, config update gated by admin auth
• Docs: docs/ pricing section + README note

Non-goals

• Fee distribution/treasury (separate issue)
• USDC or other token pricing
• Renewal fees (separate issue)
• Auctions or premium-name lists

What "done" looks like

• price_for returns deterministic tiers, e.g. len 3–4: 100 XLM-stroops scale, documented constants
• register rejects underpayment with a new RegistryError::InsufficientFee (code 6)
• Overpayment is accepted and NOT refunded (documented) or rejected — pick one, document it, test it
• Fee config update requires admin authorization
• Unit tests cover every tier boundary and both payment paths
• cargo fmt --all -- --check, clippy -D warnings, full tests pass
• WASM builds for wasm32v1-none and stays under 150 KB

Implementation guidelines

• Follow the existing pattern: validate → require_auth → mutate → write (see register in src/lib.rs)
• Reuse the workspace error style in RegistryError; add the variant at the next free code
• Use env.storage().instance() for FeeConfig (small hot config), consistent with DataKey::Count
• Attached-payment checks: use the native XLM contract address via token::StellarAssetClient/Client patterns already present in the SDK; do not introduce new dependencies
• Keep all fallible validation before any storage mutation

Verification

cargo fmt --all -- --check
cargo clippy --workspace --all-targets --locked -- -D warnings
cargo test -p sidera-registry --all-targets --locked
cargo build --locked --release --target wasm32v1-none -p sidera-registry

PR guidelines

• Get assigned before starting; do not start work on a reserved issue.
• Work in a fork; branch off upstream/main; open the PR against Sidera-Protocol/sidera-contracts:main.
• PR description must include Closes #1.
• All CI checks must pass; a human maintainer review is required (see CONTRIBUTING.md).

Metadata

• Affected crates/files: contracts/registry/src/lib.rs, docs/
• Complexity label: complexity: medium (150 base points)
• Type labels: enhancement