fix: bind VBlob data views to their arena

[coldmans]

This fixes the arena lifetime and thread-access guarantees of buffers returned by VBlob.asArenaScopedByteBuffer().

getUnsafeDataAddress() currently uses asSlice() to set the bounds of the pointer returned by vips_blob_get(). The pointer retains its global scope, so the returned buffer's scope remains alive after the owning arena closes and releases the native memory. Buffers created with Arena.ofConfined() also appear accessible from other threads.

The following reproduces the lifetime mismatch without reading freed memory:

ByteBuffer buffer;
try (var arena = Arena.ofConfined()) {
    buffer = VBlob.newFromBytes(arena, new byte[]{1, 2, 3})
            .asArenaScopedByteBuffer();
}
System.out.println(MemorySegment.ofBuffer(buffer).scope().isAlive());
// Before: true; after this fix: false

Changes

• Use reinterpret(length, arena, null) to bind the data view to the blob's arena and set its bounds. This preserves the existing cleanup ownership without copying data or registering another deallocator.
• Add six native regression tests covering access after arena closure, segment scope, confined/shared arena access, independent copies, and blobs produced by PNG encoding. Tests check scope metadata before attempting access after closure, so they fail safely on the unpatched implementation without reading freed memory.
• Add a scope check to the existing VBlobByteBufferSample so the existing sample CI also catches the regression. The native tests live in integrationTest, keeping the default unit tests runnable before libvips is installed.

Validation

Tested against base commit 1ed5dd596fb48ab8c7b69b21a28f05fc3090851e on Linux ARM64 with Temurin 25, libvips 8.18.6, and JUnit Platform Console 1.11.3.

• Compiled all core sources with javac --release 22, then ran the six new tests and the existing SenseCheckTest: 2/7 passed and 5/7 failed before the fix; 7/7 passed after it.
• The updated VBlobByteBufferSample fails its scope assertion before the fix and passes afterward.
• git diff --check passes. I have not run the full Gradle build or tested other OS/JDK combinations locally.

With the project's toolchain and libvips installed, the new regression tests can be run with:

./gradlew :core:integrationTest --tests app.photofox.vipsffm.VBlobArenaTest

Compatibility

This enforces the documented arena lifetime contract; it does not reduce native allocations or change when native memory is released. Code that previously accessed a confined arena's buffer from another thread will now be subject to the arena's thread-access checks. Such callers should use a shared arena or an independent copy.

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[lopcode]

Thank you for your contribution, and for including tests for validation. I realised there were a couple of other places with the same bug, so I included them in this PR.