fix data races in cache, resolver, qlog; add cache/zonefile/blocklist tests; update STATUS

- cache.Entry.Hits and LRU.perShard become atomics; SetCapacity now evicts
  down to the new budget; packKey fully qualifies names before packing
- resolver.Pool rewritten copy-on-write (atomic.Pointer to an immutable
  state); Upstream is immutable with shared *upstreamStats, so health
  survives a Rebind and the query path stops racing the config hook
- Pool.Exchange no longer marks a dead upstream healthy: Success was
  called before the error check, so the breaker never tripped
- qlog live ring publishes through atomic pointers
- blocklist.Holder.Get nil-safe; Builder lists each source once
- web: fix toast(string) misuse that rendered empty mutation toasts
- add unit tests for cache (with benchmarks) and zonefile
- STATUS.md: green -race integration suite, remaining failures and the
  broken bbolt snapshot round trip documented

internal/blocklist/set.go

@@ -214,13 +214,16 @@ type SourceMeta struct {
 
 // Builder accumulates rules and produces a Set.
 type Builder struct {
-	exact  map[string]*Entry
-	sub    map[string]*Entry
-	root   map[string]*Entry
-	meta   map[string]SourceMeta
-	order  []string
-	dup    int
-	counts RuleCounts
+	exact map[string]*Entry
+	sub   map[string]*Entry
+	root  map[string]*Entry
+	meta  map[string]SourceMeta
+	order []string
+	// ordered mirrors order, so a source is listed once even when both a rule
+	// and its metadata mention it.
+	ordered map[string]bool
+	dup     int
+	counts  RuleCounts
 }
 
 // NewBuilder returns an empty Builder.
@@ -286,9 +289,7 @@ func (b *Builder) Add(r Rule, source string, categories []string) {
 	}
 	m[name] = e
 	b.countAction(e, 1)
-	if _, seen := b.meta[source]; !seen {
-		b.order = append(b.order, source)
-	}
+	b.noteOrder(source)
 }
 
 // metaFor reports whether a source already contributed to this build.
@@ -326,12 +327,25 @@ func (b *Builder) uncount(e *Entry) {
 
 // NoteSource records (or updates) bookkeeping for a contributing source.
 func (b *Builder) NoteSource(meta SourceMeta) {
-	if _, seen := b.meta[meta.Name]; !seen {
-		b.order = append(b.order, meta.Name)
-	}
+	b.noteOrder(meta.Name)
 	b.meta[meta.Name] = meta
 }
 
+// noteOrder appends a source to the build order exactly once.
+func (b *Builder) noteOrder(name string) {
+	if name == "" {
+		return
+	}
+	if b.ordered == nil {
+		b.ordered = make(map[string]bool, 16)
+	}
+	if b.ordered[name] {
+		return
+	}
+	b.ordered[name] = true
+	b.order = append(b.order, name)
+}
+
 // SourceCount is the number of contributing sources.
 func (b *Builder) SourceCount() int { return len(b.meta) }
 
@@ -731,7 +745,13 @@ func NewHolder(s *Set) *Holder {
 }
 
 // Get returns the current set (nil before the first build).
-func (h *Holder) Get() *Set { return h.cur.Load() }
+// Get returns the current set, or nil when nothing has been compiled yet.
+func (h *Holder) Get() *Set {
+	if h == nil {
+		return nil
+	}
+	return h.cur.Load()
+}
 
 // Set installs a new set.
 func (h *Holder) Set(s *Set) { h.cur.Store(s) }