add unit test for HcMatchGenerator cross-slice boundary position seeding

[coderabbitai]

Summary

Add a dedicated unit test for HcMatchGenerator cross-slice boundary position seeding, analogous to dfast_inserts_tail_positions_for_next_block_matching() in zstd/src/encoding/match_generator.rs.

Background

In PR #48, a routing check was added to verify that CompressionLevel::Better switches to the MatcherBackend::HashChain backend with lazy_depth == 2. However, there is no dedicated unit test that locks down the HcMatchGenerator's slice-boundary seeding logic (backfill_boundary_positions).

The existing 8 MiB window roundtrip test exercises cross-slice behavior end-to-end, but a targeted unit test analogous to dfast_inserts_tail_positions_for_next_block_matching() would more precisely catch boundary-anchor losses in HcMatchGenerator.

Proposed Test

A new test hashchain_inserts_tail_positions_for_next_block_matching() in zstd/src/encoding/match_generator.rs should:

1. Create an HcMatchGenerator (or use a MatchGeneratorDriver reset to CompressionLevel::Better).
2. Feed a first slice with a known repeating pattern (e.g., similar to the Dfast test).
3. Call start_matching / skip_matching to process the first slice.
4. Feed a second slice with content that overlaps the boundary of the first slice.
5. Assert that a cross-block match is produced immediately at the slice boundary (no leading literals), confirming that backfill_boundary_positions correctly seeded the tail of the previous slice.

References

• PR feat(encoding): add Better compression level (zstd level 7, lazy2 strategy) #48 — feat(encoding): add Better compression level (lazy2 strategy)
• Comment: feat(encoding): add Better compression level (zstd level 7, lazy2 strategy) #48 (comment)
• Requested by: @polaz