test(06-01): add failing tests for config package TOML load, merge, validate

- 9 table-driven tests covering CFG-01 through CFG-05
- TestLoadPartialOverrideFrequency, TestLoadPartialOverrideWaveform, TestLoadBothOverrides
- TestLoadUnknownKey, TestLoadNoConfig, TestLoadExplicitMissing
- TestLoadUnknownClass, TestLoadInvalidWaveform, TestLoadAllDefaultsPresent
- Stub config.Load returns nil,nil — all tests fail (RED)

config/config.go

@@ -0,0 +1,9 @@
+package config
+
+import "github.com/netsynth/netsynth/classify"
+import "github.com/netsynth/netsynth/synth"
+
+// Load is a stub — implementation pending GREEN phase.
+func Load(configPath string) (map[classify.TrafficClass]synth.FreqConfig, error) {
+	return nil, nil
+}