Make threadLocalSupplier() return a closeable supplier and close it at build call sites

[ikaitist]

Fixes #635

What

threadLocalSupplier() caches per-thread RAVV copies in an ExplicitThreadLocal that callers could never release, leaking AutoCloseable copies (e.g. mmap-backed) for the lifetime of the supplier.

• threadLocalSupplier() now returns a ThreadLocalCopies supplier that implements AutoCloseable: close() invokes close() on every AutoCloseable copy created so far and drops the per-thread cache. Un-shared RAVVs keep returning () -> this.
• GraphIndexBuilder.build() and the merge path close the supplier in a finally block when construction completes.

How verified

• New TestThreadLocalCopies (3 tests): close() closes all per-thread copies, the supplier remains usable after close, un-shared RAVVs return the source.
• GraphIndexBuilderTest regression: 5 tests, 0 failures, 0 errors.
• mvn -pl jvector-tests -am -Dtest=TestThreadLocalCopies -DskipScalar=false -DskipSIMD=true test -> 3 tests, 0 failures (SIMD execution skipped locally: the JDK here has no jdk.incubator.vector module).

[github-actions]

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