Phase 17.2: production final-flush default for Rust opt-in indexing

[jununfly]

This was generated by AI during triage.

Parent

• PRD: Rust indexing core vertical slice #49
• Phase 15: Controlled performance gate interpretation for Rust indexing #165
• Phase 17.1: Phase 17.1: final-flush default path design and failure-safety tests #183

What to build

Implement the production temp on-disk final-flush path and make it the default write path for explicit Rust indexing. A user running --engine rust should get the production final-flush behavior without opting into an experimental mode. --sqlite-write-mode disk must remain available for debugging and fallback analysis, and memory-final-flush must remain experimental/debug rather than production default.

This issue should keep Rust opt-in. It must not make Rust the default product indexer.

Acceptance criteria

• --engine rust uses the production temp on-disk final-flush path by default.
• The staged database is only promoted after a successful Rust indexing run and validation.
• A failed Rust indexing run leaves the previous good active index intact.
• --sqlite-write-mode disk still selects the disk write path.
• memory-final-flush is not the production default and is clearly treated as experimental/debug.
• Existing TypeScript shell, resolver, MCP, and Explore behavior continue to read the Rust-produced active index.
• Tests added in Phase 17.1: final-flush default path design and failure-safety tests #183 pass without weakening their failure-safety assertions.
• No SQLite schema change is introduced.

Blocked by

• Phase 17.1: final-flush default path design and failure-safety tests #183

[jununfly]

This was generated by AI during triage.

Implemented and validated.

Completed:

• --engine rust now passes --sqlite-write-mode final-flush by default.
• final-flush uses the production temp on-disk staging database plus active-index replacement path.
• --sqlite-write-mode disk remains selectable.
• memory-final-flush remains selectable but is not the production default.
• Existing TypeScript shell/status/MCP-readable index behavior remains covered by the Rust engine CLI suite.
• No SQLite schema change.

Validation:

• cargo test --package zcodegraph-core
• npm run build
• npx vitest run __tests__/rust-index-engine-cli.test.ts
• npx vitest run __tests__/rust-indexing-experiment.test.ts

Artifact:

• docs/benchmarks/2026-06-17-rust-indexing-core-phase-17-validation-and-decision.md