add dbx

[KARTIKrocks]

Added

• dbx — New dbx package: lightweight, generic row scanner for database/sql — eliminates scan boilerplate while keeping full SQL control
• dbx — SetDefault(db) sets the package-level connection once at startup; all functions use it automatically
• dbx — WithTx(ctx, tx) returns a context that overrides the default with a transaction — all dbx calls in that context use the tx
• dbx — QueryAll[T](ctx, query, args...) scans all rows into []T; returns empty slice for no rows
• dbx — QueryOne[T](ctx, query, args...) scans the first row into T; returns errors.CodeNotFound for no rows
• dbx — Exec(ctx, query, args...) executes non-returning queries with errors.CodeDatabaseError wrapping
• dbx — Q-suffixed variants (QueryAllQ, QueryOneQ, ExecQ) accept sqlbuilder.Query directly
• dbx — Struct mapping via db:"column_name" tags; db:"-" to skip; untagged fields ignored
• dbx — Pointer fields (*string, *int, etc.) handle SQL NULLs naturally
• dbx — Column matching is order-independent; unmatched result columns silently discarded
• dbx — Embedded struct support (including exported pointer embeds like *Base)
• dbx — Reflection-based type mapping cached per-type via sync.Map (built once per process lifetime)