fix: support all non-negative int64 deletion vector positions

[wirybeaver]

Summary

Use sparse ordered high-32-bit buckets to represent every non-negative int64_t deletion-vector position without storage proportional to the largest key.

• preserve portable Roaring/Puffin encoding and serialize populated buckets in order

• avoid overflow when a position-delete range ends at INT64_MAX; document the half-open endpoint limitation

• retain fixed framing-size prechecks before allocation/output creation, without adding a configurable test-only writer factory

• make emptiness checks constant-time under the nonempty-bucket invariant and reuse the computed framing length during serialization

DV-PR-2 in #916. Based directly on main at 3c5715c; no fixture-only or footer-validation dependency. Existing independent raw Roaring fixtures remain, but the standalone fixtures from #913 are not included.

Test plan

GCC 14 focused checks on the four-feature integration tree based on 3c5715c; the final changed branch sources were verified to match the tested tree.

• util_test --gtest_filter='RoaringPositionBitmap*:*PositionDeleteIndex*:*PositionDeleteRangeConsumer*': 64 passed
• data_test --gtest_filter='DVWriterTest.*': 11 passed

Existing sparse roundtrip coverage also checks restored cardinality; no new exhaustive boundary matrix was introduced.

• Repository pre-commit checks and git diff --check: passed