feat(stablecoin): add stability fee accrual

[3esmit]

Closes #97

Summary

Adds stability-fee accrual support to the Stablecoin Program by storing a
per-position fee accumulator snapshot and a program-global stability fee
accumulator. Debt helpers accrue global fees first, settle the position against
the current accumulator, then apply debt changes so collateralization checks use
fee-adjusted debt.

What Changed

• Added StabilityFeeState with stability_fee_accumulator,
  stability_fee_rate, and last_fee_update_timestamp.
• Added Position.fee_accumulator while keeping nominal debt_amount.
• Added fixed-point continuous-compounding helpers and position accrual helpers.
• Updated collateralization helpers to use accrued debt and redemption price.
• Added initialize_stability_fee_state so positions snapshot an initialized
  global accumulator at creation.
• Regenerated the Stablecoin Program IDL.

Scope

This only implements the stability-fee accrual shape requested by issue #97.
It does not add debt-generation or repayment instructions, does not read oracle
accounts directly, and does not add a fee-rate update instruction.

Validation

cargo +nightly fmt --all -- --check
taplo fmt --check .
RISC0_SKIP_BUILD=1 cargo +1.94.0 clippy --workspace --all-targets -- -D warnings
RISC0_DEV_MODE=1 cargo +1.94.0 test --workspace --exclude integratioan_tests
RISC0_DEV_MODE=1 cargo +1.94.0 test -p integration_tests
cargo +1.94.0 build -p idl-gen --release
# full IDL drift loop from the repository guide

[Copilot]

Pull request overview

This PR adds stability-fee accrual plumbing to the Stablecoin program by introducing a program-global fee accumulator state (singleton PDA) and per-position accumulator snapshots, alongside fixed-point continuous compounding helpers to ensure collateralization checks reflect accrued debt.

Changes:

• Added StabilityFeeState singleton PDA initialization flow and required it during open_position so new positions snapshot the global accumulator.
• Implemented fixed-point continuous-compounding math + helpers to accrue global fees, accrue positions, and apply debt changes after fee accrual (with saturation/overflow handling).
• Introduced shared oracle_core primitives and a mock_oracle program + IDLs to standardize oracle price-account shapes for LEZ consumers/tests.

Reviewed changes

Copilot reviewed 29 out of 32 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
stablecoin/src/tests.rs	Extends program tests to cover stability-fee state init and ensures open_position snapshots the global accumulator.
stablecoin/src/open_position.rs	Updates open_position to require the initialized global fee-state PDA and snapshot its accumulator into new positions.
stablecoin/src/lib.rs	Exposes the new initialize_stability_fee_state module from the stablecoin program crate.
stablecoin/src/initialize_stability_fee_state.rs	Adds an instruction implementation that initializes the global stability-fee state PDA (rate + initial timestamp).
stablecoin/methods/guest/src/bin/stablecoin.rs	Adds guest entrypoints for the new instruction and extends open_position to accept the fee-state account.
stablecoin/methods/guest/Cargo.toml	Adds oracle_core as a path dependency for IDL generation / type availability.
stablecoin/methods/guest/Cargo.lock	Updates the guest lockfile to reflect the new dependency graph.
stablecoin/core/src/tests.rs	Adds unit tests for fee-growth math, global/position accrual, debt-change helpers, and collateralization checks.
stablecoin/core/src/stability_fee.rs	Introduces core stability-fee math/state: accumulator, continuous compounding, accrual helpers, and collateralization checks.
stablecoin/core/src/lib.rs	Wires stability-fee APIs/types into stablecoin_core, adds PDA derivation/verification helpers, and re-exports oracle helpers/types.
stablecoin/core/Cargo.toml	Adds oracle_core dependency for shared oracle account/feed types.
oracle/core/src/lib.rs	New shared crate entrypoint exposing oracle price-account and feed validation APIs.
oracle/core/src/account.rs	Defines the canonical OraclePriceAccount data layout with Borsh encode/decode helpers.
oracle/core/src/feed.rs	Implements consumer-side feed validation (validate_price_account) and a swappable PriceFeed interface.
oracle/core/src/tests.rs	Adds tests for oracle account encoding length and feed validation behaviors (mismatch, stale, zero price, future timestamp).
oracle/core/Cargo.toml	Introduces the oracle_core crate manifest and dependencies.
mock_oracle/src/lib.rs	New mock oracle program crate exposing set_price and tests.
mock_oracle/src/set_price.rs	Adds a mock oracle instruction implementation to write canonical oracle prices into an authorized account.
mock_oracle/src/tests.rs	Adds tests covering initialization/update behavior and validation panics for the mock oracle program.
mock_oracle/core/src/lib.rs	Adds mock_oracle_core instruction/data types and conversion into OraclePriceAccount.
mock_oracle/core/Cargo.toml	Introduces the mock_oracle_core crate manifest.
mock_oracle/Cargo.toml	Introduces the mock_oracle_program crate manifest.
mock_oracle/methods/src/lib.rs	Adds host-side embedding for the mock oracle guest ELF via generated constants.
mock_oracle/methods/build.rs	Adds build script to embed RISC Zero methods.
mock_oracle/methods/Cargo.toml	Adds the mock oracle methods crate manifest and RISC0 build configuration.
mock_oracle/methods/guest/src/bin/mock_oracle.rs	Adds the mock oracle guest entrypoint and instruction wrapper for set_price.
mock_oracle/methods/guest/Cargo.toml	Adds the mock oracle guest manifest (incl. dependencies for guest wrapper + oracle types).
mock_oracle/methods/guest/Cargo.lock	Adds the mock oracle guest lockfile.
Cargo.toml	Registers new workspace members (oracle/mock-oracle) and updates workspace dependency versions.
Cargo.lock	Updates workspace lockfile for new crates and dependency graph changes.
artifacts/stablecoin-idl.json	Regenerates Stablecoin IDL to include the new initialize instruction and updated account shapes.
artifacts/mock_oracle-idl.json	Adds generated IDL for the new mock oracle program.

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[3esmit]

This PR is branched in top of #109
The commit about this PR is 539e70c