integrate: agent/nlboot-boot-menu-proof (deconflict with main)

[mdheller]

Merges current origin/main (was behind 101 commits) into agent/nlboot-boot-menu-proof. Integrates the branch's signed boot_menu contract on top of main's current module structure. No regressions to main.

Conflict resolutions

• src/nlboot/__init__.py (load-bearing) — Union of imports. Main refactored __init__ to also re-export .device and .enrollment symbols (DeviceClaim, EnrollmentRegistry, etc.); the branch expanded the .protocol import to add BootMenu and BootMenuEntry. Kept both: main's .device/.enrollment imports and the branch's expanded .protocol import (main's protocol import + BootMenu + BootMenuEntry). The __all__ list had already auto-merged as the correct union (both new boot-menu names and all device/enrollment names present). Verified both classes exist in the merged protocol.py (BootMenuEntry and BootMenu) and that import nlboot resolves BootMenu, BootMenuEntry, and DeviceClaim. No contradiction — favored main's structure and re-applied the branch's feature intent on top.
• README.md — Union. Kept the branch's Python-reference-planner intro bullets (including its unique validates optional signed boot_menu data for boot-picker / PXE-style recovery and rollback parity line) followed by main's "Stance" / "What is implemented now" / "What is still intentionally gated" sections. The branch's detailed SignedBootManifest may also include boot_menu: protocol-object documentation auto-merged cleanly into main's "Protocol objects" section and is preserved.

src/nlboot/protocol.py (branch's 158-line BootMenu/BootMenuEntry + boot-menu validation additions) and tests/test_protocol.py (boot-menu tests) auto-merged cleanly — main did not touch those regions.

Validation (offline)

• PYTHONPATH=src python3 -m pytest -q → 39 passed (35 from main + 4 boot-menu tests)
• import nlboot smoke test → all expected symbols resolve
• python3 tools/validate_lifecycle_contracts.py → OK
• No conflict markers remain.

To confirm

• No product decision required. The boot_menu contract is additive and side-effect-free (planning only, execute=false). Confirm the union import ordering in __init__.py is acceptable to your style conventions.