Adds the `open_position` instruction to the Stablecoin Program. The instruction
claims a per-owner `Position` PDA, initializes a collateral vault token holding
via a chained `Token::InitializeAccount` under the vault's PDA authority, and
moves `collateral_amount` from the user's holding into the vault with a chained
`Token::Transfer`. `Position` is persisted with `collateral_amount` and
`debt_amount = 0`; the debt path is deferred to `generate_debt`.
- Add `Position` struct, `OpenPosition` instruction variant, and
`compute_position_pda{,_seed}` / `compute_position_vault_pda{,_seed}` helpers
in `stablecoin_core` with domain-separated PDA seeds.
- Implement `open_position::open_position` mirroring the ATA `create` and AMM
`new_definition` patterns: authorization and uninitialized-state asserts, PDA
verification, and same-transaction chained `InitializeAccount` + `Transfer`.
- Wire the new instruction through the SPEL guest and regenerate the stablecoin
IDL artifact.
- Cover the happy path, all assertion paths, and PDA determinism /
non-collision in 11 new unit tests.
This updates the spel dependency, which introduces a breaking change.
To make reviewing changes easier from other changes, this update comes
in a separate commit.
