Self-host: bootstrap compile probe script + inventory gaps

[PurHur]

Goal

Add script/bootstrap-selfhost-compile-probe.php that:

1. Bundles test/selfhost/compiler_minimal/main.php
2. Runs php bin/compile.php -l and full -o until first failure
3. Prints first Unsupported expression / Unknown * / JIT error for inventory

Update docs/bootstrap-inventory.md compiler_blockers section from live probe.

Acceptance

• Script exit 0 when compile succeeds; non-zero prints actionable next lowering target
• Documented in bootstrap-selfhost.md