* perf(cache): drop OS page cache after disk cache reads
After reading from the on-disk chunk cache, advise the kernel via
FADV_DONTNEED to release the corresponding page cache pages. This
prevents double-caching the same data in both user-space and kernel
page caches, freeing RAM for other uses on systems with large disk
caches.
* fix(cache): guard dropReadCache against zero length and invalid fd
A zero-length fadvise is interpreted as "to end of file" on Linux,
which would inadvertently drop the page cache for the entire remainder
of the cache volume. Also check fd >= 0 to avoid unnecessary syscalls
when the backend file is closed.
* perf(cache): only apply FADV_DONTNEED for reads >= 1 MiB
For small needle reads the syscall overhead outweighs the memory
savings, and the kernel page cache is more beneficial for warm data.
Restrict fadvise to reads of at least 1 MiB where the freed page
cache is meaningful.
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