fix: auto-load BPF upgradeable programs (#240)

[alex-sumner]

Fixes #240

Summary

When programs are deployed via BPF Loader Upgradeable, only the ProgramData account gets written to the ExecutionRecord, not the Program account. This causes subsequent transactions to fail.

Changes

• Set executable flag for Program accounts in sync_accounts()
• Add load_all_existing_programs() to scan and load uncached programs
• Auto-load programs during transaction execution if missing from cache
• Sync BPF loader accounts even when not writable in execute_tx_helper()
• Add load_existing_programs() public API
• Add tests

Test Plan

• cargo test --package litesvm --test bpf_upgradeable_autoload (4 tests pass)