feat(jit): build native trace graph regions

[fffonion]

Summary

• track exact native trace exits and validate same-frame SSA imports
• fuse hot parent/side traces into native regions, including scalar cycles
• preserve dirty-local restoration, ownership boundaries, and invalidation lifetimes
• add region execution diagnostics and callable handoff regression coverage

Verification

• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all --check
• git diff --check