library-selection cache ignores reached headers; --clean does not invalidate it

[zackees]

Problem

The library-selection cache key covers .cpp contents and each library's main header, but not the project or local-library headers that the LDF scan reaches. Editing only a reached header (for example, removing an #include <X.h> from a .cpp.hpp pulled in by a unity .cpp) gives a cache hit under the same key, so the stale selection is reused.

fbuild build --clean does not clear ~/.fbuild/*/cache/library-selection, so even a clean build keeps the stale selection.

Repro (seen while diagnosing #1606)

1. FastLED rp2350 project: lib/FastLED/platforms/arm/rp/ble_rp.cpp.hpp includes <BTstackLib.h>, so BTstackLib is selected.
2. Remove that include from the lib copy, then run fbuild build <proj> -e rp2350 --clean.
3. BTstackLib is still selected (daemon log: selected framework library 'BTstackLib'). After moving ~/.fbuild/prod/cache/library-selection aside, the same build drops it.

Expected

The selection cache key should include every file the scan reads, or --clean should invalidate it.