feat(config): expose charset, loc, and parseTime as configurable DSN parameters

- add Config.Charset (MYSQL_CHARSET, default "utf8mb4"): connection character set
  sent as SET NAMES during handshake; previously hardcoded
- add Config.Loc (MYSQL_LOC, default "UTC"): IANA timezone for time.Time ↔
  DATE/DATETIME conversion; previously hardcoded
- add Config.ParseTime (MYSQL_PARSE_TIME, default "true"): driver-level DATE/DATETIME
  → time.Time mapping; valid values "true"/"false"; previously hardcoded
- update DSN() to derive parameters from Config fields with empty-means-default
  semantics; existing Config literals produce identical DSN output (backward compatible)
- remove unused url.URL construction from DSN(); params now built directly via url.Values
- document collation DSN limitation in Config godoc, CLAUDE.md, RELEASE.md, CHANGELOG.md:
  go-sql-driver v1.8.x uses 1-byte handshake collation IDs (max 255); MariaDB 11.4+
  collations such as utf8mb4_uca1400_as_cs exceed that range — set collation at the
  database/table level in schema migrations instead

CHANGELOG.md

@@ -5,6 +5,24 @@ All notable changes to this module will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this module adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.9.1] - 2026-03-20
+
+### Added
+
+- `Config.Charset string` (`MYSQL_CHARSET`, default `"utf8mb4"`): connection character set, sent as `SET NAMES <charset>` during handshake. Previously hardcoded to `utf8mb4`.
+- `Config.Loc string` (`MYSQL_LOC`, default `"UTC"`): IANA timezone name used for `time.Time` ↔ `DATE`/`DATETIME` conversion. Previously hardcoded to `UTC`.
+- `Config.ParseTime string` (`MYSQL_PARSE_TIME`, default `"true"`): controls driver-level `DATE`/`DATETIME` → `time.Time` mapping. Valid values `"true"` / `"false"`. Previously hardcoded to `true`.
+
+### Changed
+
+- `Config.DSN()`: now derives `charset`, `loc`, and `parseTime` from the new Config fields instead of hardcoded literals. Empty fields fall back to their respective defaults, preserving identical DSN output for callers that do not set the new fields.
+- Removed unused `net/url.URL` construction from `DSN()`; the method now builds params directly with `url.Values`.
+
+### Notes
+
+- **Backward compatible.** Existing `Config` literals that do not set `Charset`, `Loc`, or `ParseTime` produce the same DSN as v0.9.0 (`charset=utf8mb4&loc=UTC&parseTime=true`).
+- **Collation via DSN is not supported.** `go-sql-driver` v1.8.x negotiates the collation using a 1-byte handshake ID (max 255). MariaDB 11.4+ collations such as `utf8mb4_uca1400_as_cs` carry IDs > 255 and will cause a connection error if set in the DSN. Set the desired collation in schema migrations at the database/table level.
+
 ## [0.9.0] - 2026-03-18
 
 ### Added
@@ -31,4 +49,5 @@ and this module adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.
 - The module is structurally parallel to `postgres` but uses `database/sql` types throughout; the two modules are intentionally type-incompatible.
 - MySQL error codes are matched by numeric constant via `MySQLError.Number`, not by string parsing, for stability across MySQL and MariaDB versions.
 
+[0.9.1]: https://code.nochebuena.dev/go/mysql/releases/tag/v0.9.1
 [0.9.0]: https://code.nochebuena.dev/go/mysql/releases/tag/v0.9.0

CLAUDE.md

@@ -22,6 +22,8 @@ Provides a `Component` that manages a `*sql.DB` connection pool, satisfies the `
 - **Driver import alias** (ADR-003): The `go-sql-driver/mysql` driver package name collides with the package name `mysql`. In `errors.go` it is imported as `mysqldrv` to disambiguate. In `mysql.go` it is imported with `_` for side-effect registration only.
 - **Error mapping via MySQLError.Number**: `HandleError` type-asserts to `*mysqldrv.MySQLError` and switches on `.Number`. Error codes 1062 (duplicate key) → `ErrAlreadyExists`; 1216, 1217, 1451, 1452 (foreign key violations) → `ErrInvalidInput`. `sql.ErrNoRows` → `ErrNotFound`.
 - **UnitOfWork via context injection**: Same pattern as the `postgres` module — active `*sql.Tx` is stored under `ctxTxKey{}` in the context. `GetExecutor(ctx)` returns the transaction if present, otherwise `*sql.DB`.
+- **Configurable DSN parameters (v0.9.1)**: `Config.Charset`, `Config.Loc`, and `Config.ParseTime` are optional string fields that control the corresponding DSN parameters. Empty value means "use the safe default" (`utf8mb4`, `UTC`, `true`). Existing `Config` literals that do not set these fields behave identically to v0.9.0.
+- **Collation DSN limitation**: `go-sql-driver` v1.8.x negotiates the connection collation via a 1-byte handshake ID (max 255). MariaDB 11.4+ collations such as `utf8mb4_uca1400_as_cs` exceed that range and cannot be specified in the DSN. Set collation at the schema level (database/table DDL) instead.
 
 ## Patterns
 
@@ -62,6 +64,7 @@ defer rows.Close()
 - Do not match MySQL errors by message string. Use `HandleError` which switches on `mysqldrv.MySQLError.Number`.
 - Do not add package-level `*sql.DB` variables. `mysqlComponent` is the unit of construction; use dependency injection.
 - Do not forget `defer rows.Close()` after `QueryContext` — unclosed `*sql.Rows` hold connections from the pool.
+- Do not pass a MariaDB 11.4+ collation (e.g. `utf8mb4_uca1400_as_cs`) as `Config.Collation` or any DSN parameter — the driver will fail at connect time with "unknown collation". Set collation in schema migrations at the database/table level instead.
 
 ## Testing Notes
 

mysql.go

@@ -50,6 +50,16 @@ type UnitOfWork interface {
 }
 
 // Config holds MySQL connection settings.
+//
+// DSN parameters Charset, Loc, and ParseTime default to "utf8mb4", "UTC", and
+// "true" respectively when left empty, preserving the behaviour of v0.9.0.
+// Set them explicitly when you need non-default values (e.g. Loc="Local").
+//
+// Note on Collation: go-sql-driver v1.8.x negotiates the connection collation
+// via a 1-byte handshake ID (max 255). MariaDB 11.4+ collations such as
+// utf8mb4_uca1400_as_cs carry IDs > 255 and cannot be set through the DSN
+// collation parameter. Set the desired collation at the database/table level
+// in your schema migrations instead.
 type Config struct {
 	Host            string `env:"MYSQL_HOST,required"`
 	Port            int    `env:"MYSQL_PORT"               envDefault:"3306"`
@@ -60,20 +70,38 @@ type Config struct {
 	MinConns        int    `env:"MYSQL_MIN_CONNS"          envDefault:"2"`
 	MaxConnLifetime string `env:"MYSQL_MAX_CONN_LIFETIME"  envDefault:"1h"`
 	MaxConnIdleTime string `env:"MYSQL_MAX_CONN_IDLE_TIME" envDefault:"30m"`
+	// Charset is the connection character set sent as SET NAMES <charset>.
+	// Defaults to "utf8mb4" when empty.
+	Charset string `env:"MYSQL_CHARSET" envDefault:"utf8mb4"`
+	// Loc is the IANA timezone name used for time.Time ↔ MySQL DATETIME
+	// conversion. Defaults to "UTC" when empty.
+	Loc string `env:"MYSQL_LOC" envDefault:"UTC"`
+	// ParseTime controls whether the driver maps DATE/DATETIME columns to
+	// time.Time. Valid values: "true", "false". Defaults to "true" when empty.
+	ParseTime string `env:"MYSQL_PARSE_TIME" envDefault:"true"`
 }
 
 // DSN constructs a MySQL DSN from the configuration.
+// Empty Charset, Loc, and ParseTime fields fall back to their safe defaults
+// ("utf8mb4", "UTC", "true"), matching the behaviour of v0.9.0.
 func (c Config) DSN() string {
-	u := &url.URL{
-		Scheme: "mysql",
-		User:   url.UserPassword(c.User, c.Password),
-		Host:   fmt.Sprintf("%s:%d", c.Host, c.Port),
-		Path:   "/" + c.Name,
+	charset := c.Charset
+	if charset == "" {
+		charset = "utf8mb4"
 	}
-	q := u.Query()
-	q.Set("parseTime", "true")
-	q.Set("loc", "UTC")
-	u.RawQuery = q.Encode()
+	loc := c.Loc
+	if loc == "" {
+		loc = "UTC"
+	}
+	parseTime := c.ParseTime
+	if parseTime == "" {
+		parseTime = "true"
+	}
+
+	q := url.Values{}
+	q.Set("charset", charset)
+	q.Set("loc", loc)
+	q.Set("parseTime", parseTime)
 	// go-sql-driver uses user:pass@tcp(host:port)/db?params
 	return fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?%s",
 		c.User, c.Password, c.Host, c.Port, c.Name, q.Encode())