v0.3.0

Changed

• Breaking: the query package is reorganized into the library's layer ontology. The
  statement layer is now ast — the standard-SQL AST and its renderer — and the contract
  layer moves to the new operation package: Projection (with Field and the builders),
  Directives, and the typed field-contract errors. Statements render with
  Render(d) (ast.SQL, error) in place of SQL(d) (string, []any, error); ast.SQL is the
  rendered unit (Text, Args). The list operation is Projection.List, returning the
  List envelope (Count, Page) in place of Statements.
• Breaking: the Query interface narrows to the query expressions — Select and
  Compound, the statements valid inside a CTE, a derived table, a subquery, and
  INSERT ... SELECT — enforced at compile time; the write statements never implement it.
• Rendering validates before it emits, at every scope: each statement, clause, and loop
  iteration checks the invariants decidable at its entry before its first write, so the
  outermost defect wins under the first-failure rule.

Added

• ast — the write statements Insert, Update (with Assignment), and Delete: plain
  table names, operand-lifting values, the same predicate trees for WHERE, rendering only at
  the outermost position. The Returning clause is the first declared-native feature:
  rendered only through the ReturningRenderer capability, failing with the typed
  UnsupportedFeatureError on a dialect without it — the capability pattern's second
  direction beside PagingRenderer's standard-emission override. Writer gains Column;
  Expression gains Empty.
• operation — the query side's single-row read Projection.One, resolving the field and
  its typed unknown-field error at the library; the promoted structural builders Columns
  and Fields; the promoted RecursivePath computed-field pattern; and the command side:
  Insertion (the identity-returning insert), GuardedUpdate and GuardedDelete on the
  optimistic-concurrency contract, with Guard naming the consumer's version column and
  expected version, and the Guarded command-and-check envelope in pure standard SQL.
• exec — the execution layer, the only layer touching database/sql at runtime: Scan, the
  query runners List, One, and Query over the new Session seam, and the command
  runners Insert (returning Identity), Update (returning the deterministic new
  version), and Delete, mapping guard outcomes to sql.ErrNoRows and
  ErrVersionMismatch and routing every driver error through the dialect's MapError.
• database — the Session interface implemented by DB and the new Tx; Begin,
  ExecTx as the unit-of-work helper, and Tx.Commit mapping commit errors through the
  dialect, so violations deferred to COMMIT are classified. The error taxonomy grows the
  four constraint-class sentinels (ErrUniqueViolation, ErrForeignKeyViolation,
  ErrCheckViolation, ErrNotNullViolation), ErrVersionMismatch, and ConstraintError
  carrying the constraint name while unwrapping to both the class and the driver error.