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
