Fix cargo bench deadlock by isolating build script target directories

[levicook]

Perfect branch name! Here's a PR title and description:

PR Title:

Fix cargo bench deadlock by isolating build script target directories

PR Description:

Problem

cargo bench was hanging indefinitely during compilation, blocking development of compute unit benchmarking workflows. The issue manifested as a silent hang with no error output, making it difficult to diagnose.

Root Cause

Through process sampling and system call tracing, we discovered the hang was caused by file lock (flock) contention:

• Main cargo process holds locks on target/ directory
• Build scripts spawn cargo build-sbf subprocesses
• Subprocesses attempt to access the same target/ structure
• Deadlock occurs when both try to acquire locks simultaneously

Solution

Isolated Build Directories: Use CARGO_TARGET_DIR environment variable to build programs in isolated temp directories, preventing lock contention entirely.

Refactored Build Logic: Extracted ~200 lines of duplicated code into a shared build_solana_program_internal() helper that handles:

• Temp directory isolation
• Workspace root detection via OUT_DIR parsing
• Robust .so file discovery and copying
• Aggressive error handling with clear diagnostics

Key Changes:

• Build scripts now use isolated temp directories via CARGO_TARGET_DIR
• Extract workspace root using /target/ string splitting from OUT_DIR
• Copy all found .so files (no hardcoded program name assumptions)
• Fail fast with clear error messages instead of silent failures
• Fix .so file paths to use correct sbf-solana-solana/release/ location

Impact

✅ Eliminates cargo bench deadlock - Builds complete reliably without hanging
✅ Enables compute unit benchmarking - Unblocks CU measurement and optimization workflows
✅ Improved maintainability - Consolidated build logic, easier to debug and extend
✅ Better error handling - Clear diagnostics instead of mysterious include_bytes! failures

Testing

• cargo build --workspace completes successfully
• cargo bench cu_bench_spl_transfer --features cu_bench runs without hanging
• Both Anchor and Pinocchio programs build and load correctly
• No scattered target directories or build artifacts

This change provides a solid foundation for building intelligent transaction composition tools based on reliable compute unit measurements.