build: prefer mainline llama.cpp checkout

[bitnom]

Draft replacement for the mainline checkout preference.

Changes:

• Prefer llama.cpp-mainline over llama.cpp in monorepo and sibling layouts.
• Preserve WWAMA_LLAMA_CPP_DIR override behavior.
• Watch candidate parents so a newly added preferred checkout invalidates a fallback build.
• Correct the repository-layout documentation.
• Add resolver-order coverage.

This remains a draft until the base Windows build fixes and multi-platform validation are complete.