… rowset timestamps

The file cache TTL flow used inconsistent expiration rules across write, read,
and warmup paths. Query paths clamped expired values to 0, while writer and
some warmup paths could still produce non-zero absolute expiration timestamps.
Pending rowsets also delayed persisting newest_write_timestamp, which let cache
writes use a different time base from later reads and warmup tasks.

This change adds a shared helper to calculate file cache expiration with
validation, overflow protection, and expired-value clamping. It reuses the same
logic in rowset read, rowset write, and cloud warmup paths so the same cache
hash gets the same expiration_time consistently.

This change also persists newest_write_timestamp into pending/prepared rowset
meta during initialization when the context already provides a valid value.
That keeps import/write, query, and warmup flows aligned on the same timestamp
base and avoids generating different TTL cache directories for the same object.