Alias-first architecture for zero-downtime rebuilds

[kapral18]

Closes #132

Sibling PR (MCP server): elastic/semantic-code-search-mcp-server#39

Summary

• Make the index name a stable alias.
• On --clean, build new backing indices and atomically swap aliases (no downtime).
• Store file-level metadata in <alias>_locations and keep chunk docs content-deduped.

Model (alias-first)

<alias>            (alias)  ->  <alias>-scsi-<id>              (backing chunk index)
<alias>_locations  (alias)  ->  <alias>-scsi-<id>_locations    (backing locations index)
<alias>_settings   (index)      (commit state + maintenance lock)

Clean rebuild flow

acquire lock  ->  index into new backing indices  ->  swap aliases  ->  (optional) delete old backing  ->  release lock

Test Plan

• npm run lint (pass)
• npm run build (pass)
• npm run test:unit (pass)
• npm run test:integration (pass)