feat(search): expose semantic index build timings

[Suknna]

Summary

• Add elapsed_ms and stage_elapsed_ms to semantic index build progress/status.
• Reuse the existing semantic progress/status pipeline and debounced status_changed push frame.
• Keep the change observational only: no storage format, search behavior, or UI changes.

Context

Related issue: #81

stage_elapsed_ms starts when AFT first observes a stage transition in semantic progress events. This keeps the implementation small and avoids adding a new metrics subsystem before deeper performance work.

Verification

• cargo fmt -p agent-file-tools -- --check
• cargo check -p agent-file-tools
• cargo test -p agent-file-tools status_exposes_semantic_build_timings
• cargo test -p agent-file-tools semantic_progress_event_pushes_status_snapshot
• cargo test -p agent-file-tools building_status_returns_lexical_fallback_results
• cargo test -p agent-file-tools natural_language_auto_falls_back_to_grep_while_semantic_builds
• cargo test -p agent-file-tools lexical_only_fallback_reports_more_available_when_capped_or_over_top_k

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Summary by cubic

Expose semantic index build timings (total elapsed_ms and per-stage stage_elapsed_ms) in progress events and the status API to improve observability while indexing. Observational only; no changes to search behavior, storage, or UI. Related to #81.

• New Features
  • Add elapsed_ms and stage_elapsed_ms to SemanticIndexEvent::Progress and surface them in the status JSON and debounced status_changed push frames.
  • Track stage transitions to compute stage_elapsed_ms and reuse the existing progress pipeline.
  • Update configure/build flow to emit timings; warmup and search handlers accept the new fields without behavior changes.

^{Written for commit 638b4c1. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

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