fix: anthropic tool-call round-trip, cache zero-hit parity, round-robin load balancing

anthropic.lua v3.0.0:
- Issue 1: tool_result/tool_use round-trip
- Issue 3: thinking default OFF (opt-in via extra_body.thinking)
- Issue 4: tool_choice mapping
- Issue 5: collect_blocks preserves unknown part types
- message_stop no longer emits done=true (was overwriting tool_calls finish_reason)
- cache_read_input_tokens normalized even at 0

gemini.lua:
- transform_response was missing cachedContentTokenCount

openai.lua (Issue 6):
- transform_error handles flat envelopes, nginx HTML, bare text

chat.go mergeUsage:
- Keep PromptTokensDetails even when CachedTokens=0

scheduler.go:
- Remove sort.SliceStable by Pref; round-robin cursor is the only LB mechanism

provider.go ModelAvailable:
- Also check Pref() > prefMin, persistently failing slots exit cands

presets.go:
- 17 built-in source templates

Tests: 6 new test functions, 2 updated for new semantics

internal/config/store.go

@@ -138,6 +138,38 @@ func (s *Store) DeletedAdapters() map[string]bool {
 	return out
 }
 
+// SeedTemplates adds preset templates that the store has never seen before.
+// A preset is seeded at most once: its name is recorded in PresetTemplates so
+// a user who edits or deletes it never gets it silently restored on restart.
+func (s *Store) SeedTemplates(presets []SourceTemplate) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	seen := make(map[string]bool, len(s.data.PresetTemplates))
+	for _, n := range s.data.PresetTemplates {
+		seen[n] = true
+	}
+	existing := make(map[string]bool, len(s.data.SourceTemplates))
+	for _, t := range s.data.SourceTemplates {
+		existing[t.Name] = true
+	}
+	added := false
+	for _, p := range presets {
+		if p.Name == "" || seen[p.Name] {
+			continue
+		}
+		s.data.PresetTemplates = append(s.data.PresetTemplates, p.Name)
+		added = true
+		if existing[p.Name] {
+			continue // user already has a template by this name: never overwrite
+		}
+		s.data.SourceTemplates = append(s.data.SourceTemplates, p)
+	}
+	if !added {
+		return nil
+	}
+	return s.persistLocked()
+}
+
 // UpsertTemplate adds or replaces a source template and persists.
 func (s *Store) UpsertTemplate(t SourceTemplate) error {
 	s.mu.Lock()