Ship Connection adapters for the common drivers as optional subpath
imports: @shaferllc/keel/db/d1 (d1Connection), /db/pg (pgConnection, any
node-postgres-compatible client incl. @neondatabase/serverless), and
/db/libsql (libsqlConnection). Each duck-types its driver and imports
none, so core stays dependency-free and nothing is bundled until imported.
Adds src/db build target + package subpath exports.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Tn7UJG3uSLwPcWaKo9gHgC