refactor(indexer): unify build/update/rebuild into single parameterized _run method

[jafreck]

Summary

Replaces three near-identical private methods (_build, _update, _baselineRebuild) in IndexBuilder with a single _run(mode) method using a discriminated union type.

Motivation

The three methods duplicated:

• DB open/close lifecycle
• Pipeline stage construction (nearly identical arrays)
• PipelineContext construction (same 15+ fields)

The only differences were mode-specific: layer, generation, changedFiles, and post-run logging.

Changes

• Added BuildMode discriminated union type: 'build' | 'update' | 'rebuild'
• Unified all three methods into _run(mode: BuildMode)
• Public methods (build(), update(), baselineRebuild()) are now one-liner delegates
• Net: -71 lines (471 → 400)

Testing

All existing tests pass with identical results to main (17 pre-existing failures unchanged).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.57%. Comparing base (1f38621) to head (171bc2d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #438      +/-   ##
==========================================
- Coverage   89.58%   89.57%   -0.01%     
==========================================
  Files          83       83              
  Lines        9641     9634       -7     
  Branches     2987     2995       +8     
==========================================
- Hits         8637     8630       -7     
  Misses       1004     1004              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.