RetroPick market_engine v4

Pyth-first implementation pass for the persistent-vault rolling-epoch market engine.

Highlights:

• lock_epoch and resolve_epoch now consume Account<PriceUpdateV2> directly
• template stores Pyth feed IDs as [u8; 32]
• explicit refund_mode / claimable state on Epoch
• full-claim-only semantics in v1
• claims/refunds ring-fenced into ClaimsVault
• settlement fees moved into FeeVault

v5 additions

• Pyth-first oracle adapter retained from v4
• hardening pass on account relationships and vault-owner checks
• Rust unit tests added under program modules
• TypeScript integration-test scaffolding added under /tests
• live posted-Pyth local-validator suites added for lifecycle, stale-oracle, and confidence-width paths
• TEST_STATUS_V5.md added with audit-oriented completion status
• single-side mode is now explicitly modeled as one active outcome per epoch position
• in single-side mode, switch_side is a full-position flip, not a partial rebalance
• see SINGLE_SIDE_MODE.md for protocol and frontend guidance
• see RENT_OPTIMIZATION_V5.md for the account-layout refactor that reduces epoch and position rent
• see tests/README.md for the local Pyth validator bootstrap and oracle test runbook