Add DepositCollateral

[3esmit]

Summary

• Add DepositCollateral to the stablecoin instruction surface and guest entrypoint.
• Move additional collateral from a user holding into the existing position vault through a chained token_core::Instruction::Transfer.
• Anchor the chained transfer target to the collateral token definition's Token Program, and reject mismatched vault/source owners.
• Require both source and vault collateral holdings to be fungible before increasing Position.collateral_amount.
• Skip zero-amount downstream transfers while keeping account validation and post-state output.
• Increase Position.collateral_amount without a collateralization check, since debt is unchanged.
• Mark the mutable deposit_collateral IDL accounts as writable and update unit/integration coverage.

Closes #90

[Copilot]

Pull request overview

Adds a new DepositCollateral instruction to the Stablecoin program, enabling users to deposit additional collateral into an already-initialized position by chaining a token_core::Instruction::Transfer into the position vault. This extends the stablecoin instruction surface area while preserving the existing “position PDA anchors ownership” model and adds both unit and integration coverage.

Changes:

• Introduce deposit_collateral host-side instruction implementation with validation (PDA address checks, token-program anchoring, fungibility checks, overflow checks) and optional zero-amount behavior (skip chained transfer but still validate and emit post-state).
• Extend the guest entrypoint + IDL to expose the new instruction and mark the relevant accounts writable/signers.
• Add unit tests and an integration flow that opens a position, deposits more collateral, then withdraws.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
programs/stablecoin/src/deposit_collateral.rs	New instruction implementation with account validation, collateral accounting update, and chained token transfer emission.
programs/stablecoin/src/lib.rs	Exposes the new deposit_collateral module.
programs/stablecoin/methods/guest/src/bin/stablecoin.rs	Adds guest instruction entrypoint for deposit_collateral with account metadata.
programs/stablecoin/core/src/lib.rs	Adds DepositCollateral to the instruction enum and improves PDA-derivation mismatch errors.
programs/stablecoin/src/tests.rs	Adds extensive unit coverage for deposit_collateral, including zero-amount and validation failure cases.
programs/integration_tests/tests/stablecoin.rs	Extends the end-to-end stablecoin flow to include a collateral deposit before withdrawal.
artifacts/stablecoin-idl.json	Updates committed IDL to include the new instruction and writable account flags.

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