session.truncate to reuse a Session for successive imports without recreating the table

[sncalvo]

Problem

The manual-control API creates a staging table, inserts, transfers, drops. If a user wants to run several imports back-to-back in the same session (same connection, same schema), they either drop+recreate (slow) or live with stale data.

Proposal

session = StagingTable::Session.new(User)
session.create_table

batches.each do |batch|
  session.truncate            # O(1), keeps table structure/indexes
  session.insert(batch)
  session.transfer
end

session.drop_table

Acceptance criteria

• truncate runs TRUNCATE <staging_table> (PG/MySQL) or DELETE FROM ... (SQLite)
• Preserves any indexes created via after_insert_indexes
• Instrumented with staging_table.truncate
• Tested against all three adapters