Adds the CreatePriceObservations instruction to the TWAP oracle program.
The instruction initialises a PriceObservations PDA for a given price
source account and time window, writing the initial tick and timestamp
as the first entry.
Key design decisions:
- Per-window accounts: each (price_source, window_duration) pair maps to
a distinct PriceObservations PDA. The window duration is baked into the
PDA seed so a single price source can support multiple TWAP windows
(24h, 7d, 30d) at independent sampling rates without sharing a buffer.
- window_duration not stored on struct: it is implicit in the PDA address.
Any reader that located the account already knows the window duration
used to derive it. Storing it would be redundant.
- Authorization is implicit: the PriceObservations PDA is derived from
the price source account ID, so is_authorized = true on the price source
proves the caller controls it without a redundant authority field.
- Impersonation is prevented by the PDA check: passing a controlled price
source with a victim's observations account ID fails immediately because
the computed PDA (from the attacker's source) does not match.
Closes#126
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.
