feat: add deletion vector compaction planning and execution

[wirybeaver]

Note

depends on #918

Summary

Deliver DV-aware compaction as one feature, combining the former standalone planner (#918) with its executor.

• form deterministic, snapshot-bound groups within partition/spec boundaries using file size and file-scoped delete pressure

• use ordinary double-precision heuristic thresholds; remove the redundant maximum-size ratio and exact binary-rational comparison machinery

• read real surviving rows through the existing scan reader, write replacement files, and atomically commit them with obsolete delete-entry removal

• preserve row lineage, reject stale/duplicate plans, retain shared Puffin objects, and safely handle output cleanup and unknown commit states

• treat empty planner results as no-ops, centralize execution-result handling, and reuse shared cleanup and native stream ownership

• consolidate before/after-creation fault injection and cover successful multi-file compaction together with a fully deleted group

DV-PR-4 in #916. Depends only on DV-PR-1 / #919 for shared cleanup behavior and end-to-end test setup. The complete compaction feature is one commit above #919. No prerequisite commits from #913, #914, #915, or the separate #918 remain.

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.

• data_test --gtest_filter='*Compaction*': 14 passed

Coverage includes empty-plan no-op, successful multi-file/two-group compaction with a fully deleted group, exact surviving rows/lineage, shared Puffin retention, stale/duplicate plans, atomic commit failure, cleanup retry, and before/after-creation failures. Planner logic was unchanged in this refactor.

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