Add configurable interest rate models

[TUPM96]

Closes #357

Summary

• Adds pluggable interest rate model infrastructure for lending with Linear, Kink, Jump, and Exponential formulas.
• Stores the active model in InterestRateConfig and adds a governance update API through update_interest_rate_model.
• Wires the lending interest-rate/risk-monitor modules into the contract API and re-exports config/model types for generated clients.
• Extends the hello-world interest-rate module with the same model choices and switch API.
• Adds formula tests, lending client model-switch tests, docs, and benchmark entries for each model.

Validation

• cargo check --manifest-path stellar-lend/contracts/lending/Cargo.toml --lib
• cargo test --manifest-path stellar-lend/contracts/lending/Cargo.toml interest_rate --no-run (still blocked by pre-existing borrow_test/math_safety_test harness errors unrelated to this change)
• cargo check -p stellarlend-benchmarks (still blocked by pre-existing hello-world duplicate definitions/stale tests)
• git diff --cached --check

Notes

• Existing variable-rate debt reads the active model at accrual time, so model switches migrate lazily.
• Existing stable-rate debt keeps its stored stable_rate_bps snapshot; new stable borrows use the active model.

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for selectable, prebuilt interest-rate models (linear/kink/jump/exponential) and exposes endpoints to query/switch the active model, along with tests, benchmarks, and docs.

Changes:

• Introduces InterestRateModelKind and model-specific borrow-rate calculations with shared clamping logic.
• Adds contract APIs + tests to query/update the active interest rate model.
• Extends benchmark suite and adds documentation for the available models.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
stellar-lend/contracts/lending/src/lib.rs	Exposes interest-rate model types and adds contract entrypoints for model querying/updating.
stellar-lend/contracts/lending/src/interest_rate.rs	Implements multiple interest-rate model formulas, config updates, and event publication.
stellar-lend/contracts/lending/src/interest_rate_test.rs	Updates event assertions and adds coverage for switching between models.
stellar-lend/contracts/lending/src/borrow.rs	Updates config update construction to include the new model field.
stellar-lend/contracts/lending/interest_rate_models.md	Documents supported rate models, behavior, migration notes, and benchmarks.
stellar-lend/contracts/hello-world/src/lib.rs	Adds APIs to read/switch the active interest-rate model.
stellar-lend/contracts/hello-world/src/interest_rate.rs	Adds model enum and model-based rate calculation + admin model switching.
stellar-lend/benchmarks/src/lending_benchmarks.rs	Adds benchmarks for switching interest-rate models.
stellar-lend/benchmarks/src/framework.rs	Adds budget entries for the new interest-rate model benchmarks.

Comments suppressed due to low confidence (1)

stellar-lend/contracts/lending/src/interest_rate.rs:76

• Using Option<u32> for model makes the API less type-safe and forces callers to cast enum variants (and allows accidental invalid codes). Prefer pub model: Option<InterestRateModelKind> in InterestRateConfigUpdate and accept/serialize the enum directly; this removes the need for from_code at the boundary and reduces invalid-input surface area.

pub struct InterestRateConfigUpdate {
    pub model: Option<u32>,
    pub base_rate_bps: Option<i128>,
    pub kink_utilization_bps: Option<i128>,
    pub slope_bps: Option<i128>,
    pub jump_slope_bps: Option<i128>,
    pub rate_floor_bps: Option<i128>,
    pub rate_ceiling_bps: Option<i128>,
    pub spread_bps: Option<i128>,
}

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