Add JMH benchmarks comparing read I/O strategies under memory pressure

[neoremind]

Adds JMH benchmarks to compare read I/O strategies in memory constrained scenario, related to #16044.

I/O strategies tested:

• mmap no madvise
• mmap + MADV_NORMAL + MADV_WILLNEED
• mmap + MADV_RANDOM
• mmap + MADV_RANDOM + MADV_WILLNEED
• FFI pread(2) via Panama
• FileChannel + DirectByteBuffer (simulates NIOFSDirectory)
• FileChannel + HeapByteBuffer
• O_DIRECT

Thread counts: 1, 4, 8, 16.

How to run

dd if=/dev/urandom of=/path/to/pread-bench-16G.dat bs=1M count=16384

java -jar lucene/benchmark-jmh/build/benchmarks/lucene-benchmark-jmh-11.0.0-SNAPSHOT.jar RandomReadIOBenchmark \
  -jvmArgs "--enable-native-access=ALL-UNNAMED -Xms2g -Xmx2g -Dbench.file=/path/to/pread-bench-16G.dat -Dbench.fileSizeMB=16384" \
  -p readSize=16384 -p readsPerOp=16

[jimczi]

madvise needs a page-aligned start address, so passing the raw random offset here makes it return EINVAL and do nothing — and since rc is discarded, it fails silently. On a c6id.4xlarge strace shows ~5117/5181 of these calls returning -1 EINVAL, so the prefetch never actually runs and the prefetch rows end up identical to plain mmap. The real Directory avoids this because MemorySegmentIndexInput#advise rounds the start down to the page first.

Suggested fix (mirrors what the Directory does):

Suggested change

	MemorySegment slice = mmapSegmentNormal.asSlice(offsets[i], readSize);
	int rc = (int) POSIX_MADVISE.invokeExact(slice, (long) readSize, MADV_WILLNEED);
	// madvise needs a page-aligned start address, otherwise it returns EINVAL and is a no-op.
	long offsetInPage = (mmapSegmentNormal.address() + offsets[i]) % ALIGNMENT;
	long aoff = offsets[i] - offsetInPage;
	long alen = readSize + offsetInPage;
	MemorySegment slice = mmapSegmentNormal.asSlice(aoff, alen);
	int rc = (int) POSIX_MADVISE.invokeExact(slice, alen, MADV_WILLNEED);
	assert rc == 0 : "posix_madvise failed: " + rc;

Same change is needed in doMmapMadvRandomBatchedPrefetch (against mmapSegmentMadvRandom). With this, single-threaded mmap+prefetch goes from ~0.15 → ~4.2 ops/ms cold (≈7× pread at T01, ~device saturation). Might be worth asserting on rc at the other madvise call sites too so this can't silently regress again.