Rust-hybrid wall-clock A/B v2: implement one bounded candidate

[jununfly]

This was generated by AI during triage.

Context

Parent plan: docs/plans/2026-06-19-rust-hybrid-default-indexing-wall-clock-ab.md

Blocked by: #291

This issue implements the single bounded optimization candidate selected by #291. Do not mix multiple candidates into one change. If #291 records no-go, close this issue with that conclusion instead of forcing an implementation.

Scope

Target only the rust-hybrid default full-index source path. Production-code changes are allowed only for the candidate selected by #291.

If the selected candidate touches TypeScript finalization/reference resolution, keep it to low-semantic-risk mechanics such as diagnostics, batching, duplicate lookup removal, or write path cleanup. Do not change every-reference disambiguation semantics.

Acceptance Criteria

• Confirm Rust-hybrid wall-clock A/B v2: baseline profile and candidate selection #291 selected exactly one bounded optimization candidate before changing code.
• Implement only that candidate.
• Add targeted deterministic tests for changed behavior.
• Do not optimize TypeScript-only standalone indexing.
• Do not add new language/framework coverage.
• Do not change SQLite schema.
• Do not mix unrelated cleanup or secondary optimization candidates.
• Update CHANGELOG.md under ## [Unreleased] if production code changes.
• Do not update README by default.
• Do not run agent sufficiency A/B unless the candidate changes graph semantics or indexed results.
• Do not run packaged/release smoke unless the touched files affect CLI launcher, packaging, status, doctor, or release workflow paths.

[jununfly]

This was generated by AI during triage.

Completed the single bounded candidate selected by #291.

Implemented:

• Rust core extraction now reuses one tree-sitter parser per source language during a full-index run instead of constructing/configuring a fresh parser for every file.
• Parser grammar selection, extracted graph semantics, fallback behavior, and default user behavior are unchanged.

Validation:

• cargo test --package zcodegraph-core rust_index_extracts_mixed_languages_with_reused_parsers — passed.
• cargo test --package zcodegraph-core — 26 passed.
• npm run build — passed.
• cargo build --package zcodegraph-core — passed.

CHANGELOG was updated under ## [Unreleased]. README was not changed. Packaged/release smoke and agent sufficiency A/B were not run because this did not touch release paths or graph semantics.

After-profile and keep/rollback/no-go decision are recorded in #293 and docs/benchmarks/2026-06-19-rust-hybrid-wall-clock-ab-v2-after.md.