docs(ffe): refresh span-enrichment comments for the ObjectKVStore design

Drop stale design-doc artifacts (DG-/CR-/Pattern labels) and references to the
removed native-staging / close-span path and the old accumulate() method.

Author: pavlokhrebto

src/api/FeatureFlags/Internal/ResultMapper.php

@@ -279,7 +279,7 @@ private function exposureData($rawResult)
         // result actually carried one; a null/absent value must be left out
         // entirely rather than defaulted to 0/false. Absence alone does NOT
         // mean a runtime default was used -- a waterfall-assigned variant can
-        // also have no serial id. SpanEnrichmentRegistry::accumulate() only
+        // also have no serial id. SpanEnrichmentRegistry::record() only
         // treats an evaluation as a runtime default when BOTH serialId AND the
         // variant are absent.
         $serialId = $this->read($rawResult, array('serial_id', 'serialId'), null);

src/api/FeatureFlags/SpanEnrichmentAccumulator.php

@@ -12,9 +12,9 @@
  * system-tests assert exact parity, so any divergence (signed varint, wrong
  * sort, `[object Object]` defaults, bare-vs-JSON tag shape) breaks the chain.
  *
- * Lifecycle: one instance per root span, request-scoped. The DataDogProvider
- * constructs it lazily only when the span-enrichment gate is on (DG-005), and
- * the native close-span path flushes + clears it on root-span finish.
+ * Lifecycle: one instance per root span. SpanEnrichmentRegistry::record()
+ * lazily attaches it to the root span (via ObjectKVStore) on first evaluation
+ * and encodes its state onto the root span's meta; it is released with the span.
  */
 final class SpanEnrichmentAccumulator
 {
@@ -55,8 +55,8 @@ public function addSerialId($id)
 
     /**
      * Associate a serial id with a (hashed) subject. The targeting key is
-     * SHA256-hashed before storage (privacy contract DG-003) and is only ever
-     * called by the provider when `do_log` authorizes it.
+     * SHA256-hashed before storage (privacy: raw targeting keys are never
+     * emitted) and is only recorded when `do_log` authorizes it.
      */
     public function addSubject($targetingKey, $id)
     {
@@ -114,7 +114,7 @@ public function hasData()
     /**
      * Encode the accumulated state into the frozen `ffe_*` span tag set.
      *
-     * Output-shape contract (Pattern F):
+     * Output-shape contract (frozen):
      *  - ffe_flags_enc        => BARE base64 string
      *  - ffe_subjects_enc     => JSON-stringified object {sha256hex: base64}
      *  - ffe_runtime_defaults => JSON-stringified object {flagKey: valueStr}