Add KnownTags globalSerial reserve partition for virtual/special tags

Establishes the boundary FIRST_STORED_SERIAL=256: globalSerials [1,256) are
reserved for "virtual" tags that are specially handled (redirected to span
fields or processed by the tag interceptor) and NOT stored in the TagMap —
hand-assigned in tracer core; [256,..) are generated convention tags that ARE
stored (slotted/bucketed); 0 stays unknown/string-only. Adds isReserved()/
isStored() so setTag(long) can classify a tag by an O(1) range check (its
"needsIntercept by id") before routing to the interceptor vs the slot/bucket
store. Both core and the code generator agree on this boundary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dougqh

internal-api/src/main/java/datadog/trace/api/KnownTags.java

@@ -39,6 +39,27 @@ public static int nameHash(long tagId) {
     return (int) tagId;
   }
 
+  /**
+   * globalSerial partition. {@code [1, FIRST_STORED_SERIAL)} is reserved for "virtual" tags that
+   * are specially handled (redirected to span fields or processed by the tag interceptor) and are
+   * NOT stored in the TagMap — these are hand-assigned in tracer core. {@code [FIRST_STORED_SERIAL,
+   * ..]} is for generated convention tags that ARE stored (slotted/bucketed). {@code globalSerial
+   * == 0} means unknown / string-only. Both core and the code generator must agree on this
+   * boundary.
+   */
+  public static final int FIRST_STORED_SERIAL = 256;
+
+  /** True if the tagId names a reserved "virtual"/specially-handled tag (not stored in the map). */
+  public static boolean isReserved(long tagId) {
+    int globalSerial = globalSerial(tagId);
+    return globalSerial > 0 && globalSerial < FIRST_STORED_SERIAL;
+  }
+
+  /** True if the tagId names a generated, map-stored (slotted/bucketed) tag. */
+  public static boolean isStored(long tagId) {
+    return globalSerial(tagId) >= FIRST_STORED_SERIAL;
+  }
+
   /**
    * Builds a tagId from its parts: {@code globalSerial} (globally unique per known tag), {@code
    * fieldPos} (the tag's slot within its span type's positional table), and the tag {@code name}