[Tracker] Improving cuvs-lucene release robustness

Java compatibility and support

• [26.12][^1] #2517
• [26.12] Keep JDK 21 compile-time support and add a CI job that builds/runs on JDK 21
• [26.12] Clearly document the JDK support policy (compile vs. run requirements)

Lucene support

• [26.12] #2410 and expand with:
  1. Restructure cuvs-lucene code to support different Lucene API for each supported version
  2. restructure cuvs-lucene build to publish one JAR per Lucene minor (cuvs-lucene-10.N-<ver>.jar)
  3. Compile against lowest patch version, test against highest patch version per minor
• [26.12] Define a policy for adding new Lucene minors and dropping trailing ones

libcuvs ↔ cuvs-java ↔ cuvs-lucene compatibility

• #1682 (Document libcuvs ↔ cuvs-java ↔ cuvs-lucene compatibility rules)
• #1020
• Add a CI matrix testing supported libcuvs × cuvs-java × cuvs-lucene combinations

API compatibility

• Publish an API compatibility policy for cuvs-java/cuvs-lucene (no guarantee across minors, Lucene-style)

Index file format compatibility

• Publish an index format backward-compatibility guarantee (readable through the next major of lucene)
• Start working on a corpus of indices from older releases and test reading them or add a harness to be able to generate old indices on demand.
• Add backward-compatibility test matrix across Lucene versions × cuvs releases

Cross-platform compatibility

• Add explicit runtime checks + error messages for missing CUDA/insufficient VRAM when opening GPU indices
• Document platform/portability limits of GPU index files (snapshot/restore across clusters)

Performance guarantees

• #2194
• #1856
• #1623
• #2195
• #2428
• #2014
• #2180 Ideally also stand up a benchmark-results dashboard
• Expand benchmarks to run across different GPUs (RTX Pro 6000, H100, A10, L40/L40S) and multiple datasets

Testing

• #2407 Expand with updates and deletes for some records
• Expand reproducible randomized testing with seeds in cuvs-lucene, surface test seeds in CI logs and treat seed-specific failures as real bugs, not flaky
• #2105

[^1]: Items marked with [26.12] are targeted for the [26.12] release.

No files or tests are named. Start by reviewing the linked issues (#2517, #2410, #1682, #1020, and the performance/testing issues), then group the checklist by compatibility, release, performance, and testing work. This tracker is done when its listed cuvs-lucene robustness requirements are completed and each checklist item is verified.