feat(stablecoin): add StabilityFeeAccumulator account type

[gravityblast]

Single-instance PDA holding the global stability-fee multiplier as (accumulated_rate_at_last_accrual, last_accrued_at). Lazy form — the current value is projected on read; the anchor only moves on accrual.

Type + PDA helpers only, no instruction wiring yet.

closes #159

[Copilot]

Pull request overview

Adds the core “global singleton” account type for the Stablecoin program’s lazy stability-fee accumulator, including deterministic single-instance PDA derivation helpers and IDL exposure. This fits the stablecoin core crate’s pattern for program-owned singleton accounts (e.g., ProtocolParameters) and enables future instructions to read/advance the accumulator without wiring instruction logic yet.

Changes:

• Introduces StabilityFeeAccumulator account type with Borsh/serde support and Data conversions.
• Adds PDA seed + PDA computation helpers for a single-instance accumulator account.
• Re-exports the new type/helpers from stablecoin_core and updates the stablecoin IDL artifact.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
programs/stablecoin/core/src/stability_fee_accumulator.rs	New singleton accumulator account type, Data conversions, PDA helpers, and unit tests.
programs/stablecoin/core/src/lib.rs	Exposes the new module and re-exports its public API from the crate root.
artifacts/stablecoin-idl.json	Adds the StabilityFeeAccumulator struct definition to the generated IDL types.

Suppressed comments (1)

programs/stablecoin/core/src/stability_fee_accumulator.rs:94

• last_accrued_at is documented as Unix milliseconds, but this test uses a seconds-scale value. Consider using a milliseconds-scale timestamp here as well to keep unit expectations consistent.

            last_accrued_at: 2_000_000_000,

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