feat: SelectKeyed scans rows into a map by column key

[sonnemusk]

Summary

Adds SelectKeyed / SelectKeyedContext (with DB / Tx / Stmt / Conn methods) so a query result can be loaded into a map[K]V keyed by a named column or db-tagged field.

var people map[string]Person
err := db.SelectKeyed(&people, "email", `SELECT first_name, last_name, email FROM person`)
// people["jmoiron@jmoiron.net"].FirstName == "Jason"

var byCode map[int]Place
err = db.SelectKeyed(&byCode, "telcode", `SELECT country, city, telcode FROM place`)

Rules

• dest must be a non-nil *map[K]V
• V must be a struct or *struct (StructScan-compatible); scannable map values are rejected
• key is a column / field name resolved the same way as StructScan (db tags + NameMapper)
• Map key type must be assignable or convertible from the key field type
• Destination map is replaced with a fresh empty map before scanning (same idea as Select resetting slice length)
• Duplicate keys: later row wins

Addresses #963 (map-by-primary-key style loading without requiring schema-level PK discovery).

Test plan

• go test -run TestSelectKeyed
• map[string]Person by email
• map[string]*Person
• map[int]Place by telcode
• errors for missing key field, empty key, non-map dest