Reject file-storage TTLs that cannot produce a finite expiration

[zackbart]

Nonfinite TTL values and overflowing expiration arithmetic serialize as null in JSON snapshots, corrupting state written by the adapter itself.

Required behavior and acceptance criteria:

• Set and compare-and-swap reject invalid expiration values before changing memory or disk. Previous data remains readable after reopening. Existing zero and finite TTL behavior stays consistent.
• Add a regression reproducing the failure and verify the applicable Node and Workers suites.

Found during the post-merge reliability sweep following #643.