feat: support Mutex and Holdout groups for web experiment

[tyiuhc]

Summary

Support Mutex and Holdout groups for web experiment.

Flags that transitively depend on a remote-evaluated flag (e.g. a Mutex or Holdout parent) are now promoted to remote evaluation via a fixed-point dependency walk. This ensures child flags wait for their remote dependencies to be fetched before bucketing runs, preventing incorrect variant assignment.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: No

---

Note

Medium Risk
Changes flag evaluation ordering and startup timing for experiments with flag dependencies or holdout/mutex flags, which can affect which variants users see on first paint.

Overview
Web experiment startup now treats local flags that depend on a remote flag (including transitive chains) as remote so variant application waits until remote parents are fetched, avoiding wrong mutex/holdout bucketing from stale parent state. Only flags promoted via that dependency walk are removed from the early local apply pass; directly remote flags that are session-cached as local still get the fast-path local apply.

When any initial flag key starts with holdout- or mutex-, start() awaits integrationManager.ready() before applying variants so bucketing has user_id / device_id from the Amplitude integration setup.

Adds a test that a depth-2 local→local→remote dependency chain is not applied on the first local applyVariants call and is applied after remote fetch.

^{Reviewed by Cursor Bugbot for commit d232bbf. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

Bugbot Autofix prepared a fix for the issue found in the latest run.

• ✅ Fixed: Local flags with remote dependencies applied prematurely
  • Added filter to exclude remoteFlagKeys members from localFlagKeys at line 218, ensuring flags with transitive remote dependencies wait for fetchRemoteFlags() before being applied.

Or push these changes by commenting:

@cursor push 59cf49554e

Preview (59cf49554e)

diff --git a/packages/experiment-tag/src/experiment.ts b/packages/experiment-tag/src/experiment.ts
--- a/packages/experiment-tag/src/experiment.ts
+++ b/packages/experiment-tag/src/experiment.ts
@@ -214,9 +214,12 @@
       ...this.config,
     });
     // Get all the locally available flag keys from the SDK.
+    // Exclude flags that were moved to remoteFlagKeys due to transitive dependencies.
     const variants = this.experimentClient.all();
     this.localFlagKeys = Object.keys(variants).filter(
-      (key) => variants[key]?.metadata?.evaluationMode === 'local',
+      (key) =>
+        variants[key]?.metadata?.evaluationMode === 'local' &&
+        !this.remoteFlagKeys.includes(key),
     );
     this.messageBus = new MessageBus();
   }

_{You can send follow-ups to the cloud agent here.}

^{Reviewed by Cursor Bugbot for commit 7b7dba7. Configure here.}