Add scalasql-zio module: ZIO interop layer

[ivan-klass]

What

Adds a new scalasql-zio module that wraps ScalaSql's JDBC DbApi/DbClient in ZIO effects — type-safe database operations and transactions with savepoints. Scala 3 only.

This ports https://github.com/typebricks/scalasql-zio (authored by me) into the main repo, converting the sbt build to Mill and moving the code under the scalasql namespace.

API

• scalasql package — PrimaryKey (zero-cost typed PKs), DbEntityName, DbLookupException (NotFound / MultipleResults), IsCovariantHK / widenK.
• scalasql.dbzio package — ZDbApi / ZDbTxnApi / ZDbClient (ZIO wrappers around DbApi / DbClient), DbOp/DbZIO helpers, Savepoint, and atMostOne / orNotFound syntax.

The subpackage is named dbzio (not zio) on purpose: scalasql.zio would shadow _root_.zio under wildcard imports.

import scalasql.dbzio.*

ZDbClient.transaction:
  for
    _    <- DbOp.run(User.insert.columns(_.email := email, _.name := name))
    user <- DbOp.runSingle(User.select.filter(_.email `=` email))
  yield user

JDBC errors are wrapped in DbException; the enclosing transaction commits on success and rolls back on failure.

Build / test

• New Mill module scalasql-zio (depends on scalasql + dev.zio::zio), mirroring the layout of scalasql-simple.
• Tests run on an in-memory SQLite DB via zio-test — no external services needed. They are picked up by the existing __.test CI step.
• The original test dependency on iron was dropped; refined Email/UserName types in tests are now plain opaque types.
• Formatted with the repo's scalafmt config and clean under __.fix (scalafix RemoveUnused).

Notes / open questions

• Publishing inherits the repo's com.lihaoyi org → artifact scalasql-zio_3. Happy to add a developer entry / adjust attribution as maintainers prefer.
• readme.md gains a short "ZIO integration" section.

🤖 Generated with Claude Code

[NPCRUS]

0.3.1 or 0.3.2 i guess, depends on how we will publish this

[NPCRUS]

• doesn't look like this should live inside dbzio
• don't know if we need it at all