RowCursor flyweight and generation guard for clj-r2dbc streaming.

Provides:
  Cursor          - protocol for type-safe access to mutable RowCursor fields.
  GenerationGuard - Row wrapper built by cursor-row; guards against stale reads
                    after the cursor advances to the next row.
  RowCursor       - shared mutable flyweight constructed once per stream* call;
                    reused for every row emitted.

Note: RowCursor is a shared mutable object. The same instance is mutated for
every row emitted by stream*. Retaining a reference to the cursor beyond the
current m/?> boundary silently returns data from a later row - no exception is
thrown. Callers that need to keep row data must either:
  (a) supply :builder-fn (recommended) to materialize an immutable value, or
  (b) call cursor-row and cursor-cache immediately within the same reduce step
      and pass them to impl/sql/row's row->map before the next row arrives.

This namespace is an implementation detail; do not use from application code.