fix: avoid writing memory-rejected disk hits back to storage

A disk-loaded entry rejected by the memory admission filter becomes a phantom entry. Under WriteOnEviction, dropping that phantom previously sent it through the storage pipe again, so a disk hit could run admission and enqueue another disk write.

Skip the pipe for phantom entries whose source is already disk. Ordinary outer-source phantoms and later evictions of memory-resident entries retain their existing writeback behavior.

The regression test verifies both that disk admission is not invoked a second time and that no keeper piece is enqueued. It fails against the previous behavior.

Validation:

• cargo test -p foyer --lib --tests
• cargo test -p foyer-memory --lib --tests
• repository CI-equivalent Clippy configurations