fix(encoding): read wrapped miniblock level counts

[lance-gatefixer]

Summary

• let block decompressors report a payload-derived value count when the encoding can determine it
• recover v6.0.1 RLE structural levels only when the decoded/header difference is an exact u16 wrap, while preserving strict corruption checks
• add a version-pinned v6.0.1 generator, historical stable-format fixture, and unit/end-to-end regressions

Root cause

Pylance 6.0.1 cast a miniblock's structural level count to u16. For list pages with a dense prefix and many trailing empty lists, the header wrapped while the RLE payload retained every level. Readers trusted the truncated count and either short-read the list column or rejected the complete payload as an RLE overflow. Current writers already prevent these oversized miniblock chunks; this change restores stable-format read compatibility for files already written.

Validation

• cargo test -p lance-encoding miniblock_levels --lib
• cargo test -p lance-encoding block_rle_infers_value_count_from_run_lengths --lib
• cargo test -p lance test_v6_0_1_miniblock_level_count_overflow --lib
• cargo fmt --all -- --check
• cargo clippy --all --tests --benches -- -D warnings
• make build from python/
• uv run make lint from python/
• local Python scan and merge-insert against the historical fixture

Fixes #6936