feat(stablecoin): rebuild repay_debt with floor-rounded debt deltas

[gravityblast]

Rebuilds repay_debt to spec §10.8. The Plan 1 scaffold subtracted amount
directly from normalized_debt_amount and trusted whatever
stablecoin_definition the caller passed.

Accounts go 4 → 7, adding stability_fee_accumulator, protocol_parameters and
clock. The decrement is now ⌊amount × FIXED_POINT_ONE / current_accumulator⌋,
rounded down per §6.3 against the accumulator projected to the clock
timestamp — debt shrinks by at most what was burned, and the rounding remainder
stays with the protocol. stablecoin_definition is pinned against
ProtocolParameters.stablecoin_definition_id, closing the gap the scaffold's
// TODO called out.

Allowed while frozen: protocol_parameters is read for the accumulator rate and
the definition binding, never for is_frozen. Repaying only improves the
protocol's position (§7).

7 new tests: the three new accounts echoed, the floor rounding at accumulator 3.0
(100 → 33), repay while frozen, an unbound stablecoin definition, overrepay
against the floored delta, and an uninitialized variant of each new global.

Sixth of eight issues in Plan 3 (#173). Stacked on #352.

closes #179

[Copilot]

🟡 Changes recommended

There are correctness/documentation gaps in the updated repay_debt documentation/tests and an error message mismatch that should be aligned with the spec/acceptance criteria for clarity and consistency.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR rebuilds the stablecoin program’s repay_debt instruction to be fee-aware per spec §10.8 by projecting the stability fee accumulator to the provided clock timestamp, computing a floor-rounded normalized-debt delta, and pinning the stablecoin definition against ProtocolParameters.

Changes:

• Update repay_debt host logic to compute debt_delta = ⌊amount × FIXED_POINT_ONE / current_accumulator⌋ (projected to clock) and validate stablecoin_definition via ProtocolParameters.
• Expand the instruction ABI from 4 → 7 accounts across host + guest wiring (adds accumulator, protocol parameters, clock).
• Add/adjust unit + integration tests and regenerate the stablecoin IDL to match the new account list.

File summaries

File	Description
programs/stablecoin/src/repay_debt.rs	Implements fee-aware, floor-rounded debt delta with protocol-parameters binding and clock-based accumulator projection.
programs/stablecoin/methods/guest/src/bin/stablecoin.rs	Updates guest entrypoint signature to pass the three new accounts through to the host function.
programs/stablecoin/core/src/lib.rs	Updates Instruction::RepayDebt documentation to reflect the new account list and semantics.
programs/stablecoin/src/tests.rs	Adds new unit tests for §10.8 behavior and updates existing repay tests for the new ABI.
programs/integration_tests/tests/stablecoin.rs	Seeds the new global accounts and updates the integration repay test to pass the new accounts.
artifacts/stablecoin-idl.json	Regenerates IDL to include the three new accounts for repay_debt.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 3
• Review effort level: Lite

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.