review/unsafeaccess jdk25

[bmarwell]

AI generated, unreviewed from me.

• Add RuntimeSupport JVM version gate for Unsafe use
• Add Unsafe-free heap array access backend
• Use HeapAccess so JDK 25+ never instantiates Unsafe

[bmarwell]

Known gaps on JDK 25+ (unsafe-free backend)

None of the performance points below are measured yet.

• Access.unsafe(): public API, so kept for compatibility; the library itself never calls it. It now throws UnsupportedOperationException on the safe backend, because object + field-offset access (the Pair example in its Javadoc) cannot be expressed without Unsafe. FFM has no equivalent for ordinary objects.
• SafeHeapAccess reads: it only implements getByte, so getLong is composed from eight byte reads. Likely slower than one Unsafe.getLong unless C2 merges the loads; most visible on short inputs and 8-byte-stride algorithms (XXH3, wyhash, metro). JDK < 25 is unchanged.
• MemorySegment.ofArray(): not used. It allocates a segment object per call, which breaks the zero-allocation guarantee unless escape analysis removes it (not guaranteed, unmeasured).
• Likely fix, not built: for byte[], a byteArrayViewVarHandle read (Java 9+, reached reflectively to keep the Java 8 build) should give a single load. Same-width array types could be indexed directly. To be decided after benchmarking.
• Direct buffers: hashBytes(ByteBuffer) uses ByteBufferAccess on the safe path. The old fast path read the native address through the internal sun.nio.ch.DirectBuffer and hashed it via Unsafe. Avoiding that internal API costs us the fast path; FFM would need a segment per call (allocation).
• Strings: String.value is a private field, which FFM cannot read; the old path used reflection plus Unsafe offsets. JDK 25+ therefore uses UnknownJvmStringHash (CharSequence reads, charAt per element): correct and allocation-free, but slower for long strings.
• FFM raw memory (hashMemory): uses the restricted MemorySegment.reinterpret, so JDK 24+ warns once unless --enable-native-access=ALL-UNNAMED is set. Tested on x86 Linux (JDK 25, 26) only; untested on AIX ppc64 / Linux ppc64le. Addresses >= 2^63 are unsupported.