[2.6] update_orderbook: handler, validation, state reconciliation, tests

[0xfave]

Implements the update_orderbook instruction (Ticket 2.6).

Changes:

• src/instruction.rs — Added UpdateOrderbookAccounts struct + TryFrom
• src/processor.rs — Full handler with authority check, PDA validation, still-delegated guard, order_count reconciliation
• tests/flux_dex_tests.rs — 3 tests: happy path, non-authority rejection, still-delegated rejection
• docs/epics/epic-2-smart-contract.md — Marked ticket 2.6 as Done

Spec deviations:

• Added still-delegated guard (reject if is_delegated == true) beyond the original spec — callers must undelegate first
• Full order-by-order diff reconciliation deferred to Epic 4 (MagicBlock SDK)

Verification:

• cargo fmt --check ✓
• cargo clippy ✓ (0 warnings)
• cargo build-sbf ✓
• cargo test — 74/74 tests pass
• CU: 6,403 (target: <20,000)

Closes #20