refactor: split dbx package into domain files

Reorganize monolithic dbx.go and utility.go into focused files by
concern, add package doc.go, and gitignore coverage.out.

Co-authored-by: Cursor <cursoragent@cursor.com>

doc.go

@@ -0,0 +1,42 @@
+// Package dbx provides database utilities built on go-jet/jet and go-toolkit/app.
+//
+// # Module lifecycle
+//
+// Call [ModuleDB] once to register the database [app.Module], then use [SQLO]
+// to access the connection pool. Only one database per process is supported.
+//
+// # Query and mutation helpers
+//
+// [Fetch], [FetchOne], [Insert], and [Update] wrap Jet [Statement] execution
+// with consistent error semantics. Must* variants return a caller-provided error
+// when no rows are found.
+//
+// # Jet column utilities
+//
+// Dialect-neutral union types ([Column], [ColumnList]) and helpers for column
+// lists ([NormalCols]) and expression building ([ExprValues]).
+//
+// Partial-update helpers ([ApplyPtr], [ApplyVal]) track changed fields for
+// repository patch logic.
+//
+// # Identifier types
+//
+// [UUID] wraps segmentio/ksuid for GraphQL APIs. [JSONB] provides map-based
+// JSON column scanning for Postgres JSONB and MySQL JSON.
+//
+// # Pointer and string utilities
+//
+// [Ptr], [Val], [TrimPtr], [StringToFilter], and related helpers for nullable
+// field handling in repository code.
+//
+// # Debug logging
+//
+// Enable [DBConfig.DebugLog] and blank-import dbxm (MySQL) or dbxp (Postgres)
+// to register Jet query debug output.
+//
+// # Dialect notes
+//
+// Use [Insert] with LastInsertId on MySQL; Postgres callers should use
+// [InsertReturning]. [InsertReturning] and [UpdateReturning] require Jet
+// RETURNING clauses.
+package dbx