Release RAM-backed splits after serialization

[PSeitz]

Summary

With in-memory indexing enabled, persisting a split copies its files to disk but leaves the returned Tantivy Index owning the RAM directory. This keeps the RAM files alive through packaging.

Reopen the persisted split from disk so the original RAM owners drop before the finalized split reaches the packager.

Changes

• Return a disk-backed Index through the existing ControlledDirectory.
• Explain when the builder's RAM handle and the shadowed original Index drop.
• Add one ownership regression test and one integration smoke test for in-memory ingestion through publication and search. The integration test sets the cached environment flag in a child process, without modifying other tests' environment.

User impact

Shorter RAM retention when QW_ENABLE_IN_MEMORY_INDEXING=true.

Verification

• Indexing library tests plus the new integration test: 234 passed. Other integration tests and the existing ignored indexing test were not run.
• make fmt and git diff --check passed.
• No production memory benchmark performed.

cargo nextest run --locked --offline \
  -p quickwit-integration-tests -p quickwit-indexing --lib \
  -E 'package(quickwit-indexing) | test(in_memory_indexing_tests)'

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-14T09:53:26.014630Z	1ca42ca	PR opened

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