Abort startup on an unknown PIXA_ environment variable (closes #133)

A variable whose name starts with PIXA_ but is neither a setting's
variable, from the list pairing each config key with its variable, nor
PIXA_CONFIG_PATH now aborts startup naming it, as an unknown config key
does. PIXA_PORT is named with a pointer to PORT. The check runs before
the config file loads, so the variables the file's env section sets are
not checked. README.md says so under Configuration.

Model: opus-5-5

internal/config/config.go

@@ -58,6 +58,7 @@ var (
 	errValueRequired     = errors.New("a value is required")
 	errValueEmpty        = errors.New("value must not be empty")
 	errUnknownConfigKeys = errors.New("unknown config keys")
+	errUnknownEnvVars    = errors.New("unknown environment variables")
 	errNotAString        = errors.New("not a string")
 	errNotAnInteger      = errors.New("not an integer")
 	errNotABoolean       = errors.New("not a boolean")
@@ -146,6 +147,11 @@ type Config struct {
 func New(_ fx.Lifecycle, params Params) (*Config, error) {
 	log := params.Logger.Get()
 
+	err := validateKnownEnvVars()
+	if err != nil {
+		return nil, err
+	}
+
 	// Look for the config file under the project name (/etc/pixa/,
 	// ~/.config/pixa/), matching the /var/lib/pixa state directory,
 	// not under the daemon name pixad.
@@ -377,6 +383,44 @@ func envVarNames() map[string]string {
 	}
 }
 
+// validateKnownEnvVars rejects environment variables whose names start
+// with PIXA_ but that are neither a setting's variable nor
+// PIXA_CONFIG_PATH, so a misspelled variable fails at startup instead of
+// being silently ignored, as validateKnownKeys does for config file keys.
+// New calls it before loading the config file, so the variables the
+// file's env section sets are not checked, just as validateKnownKeys
+// leaves that section's contents alone.
+func validateKnownEnvVars() error {
+	known := map[string]bool{"PIXA_CONFIG_PATH": true}
+
+	for _, name := range envVarNames() {
+		known[name] = true
+	}
+
+	var unknown []string
+
+	for _, entry := range os.Environ() {
+		name, _, _ := strings.Cut(entry, "=")
+
+		switch {
+		case !strings.HasPrefix(name, "PIXA_") || known[name]:
+			continue
+		case name == "PIXA_PORT":
+			unknown = append(unknown, name+" (use PORT for the port)")
+		default:
+			unknown = append(unknown, name)
+		}
+	}
+
+	if len(unknown) > 0 {
+		sort.Strings(unknown)
+
+		return fmt.Errorf("%w: %s", errUnknownEnvVars, strings.Join(unknown, ", "))
+	}
+
+	return nil
+}
+
 // lookupValue returns the value set for key and whether one is set. The
 // key's environment variable wins when it is present, even when empty;
 // its value is a string, read exactly as the same text quoted in the