[Ji-One1]

Stagehand can miss cross-origin frames that already exist when it attaches. The browser and later buyer need one shared observer attached before navigation, with a release that confirms the SDK no longer controls the browser.

What this changes

• Add prepareStagehandSession to attach Stagehand over the owned browser's CDP connection, without buyer model configuration or inference.
• Preserve the original browser-session identity for recorder lookups.
• Confirm detach through observer.context.close() with a deadline. Run SDK housekeeping separately, and always close the owned browser during session close.
• Bound initialization and cleanup, and close the browser after preparation failure.
• Refuse Stagehand flow-log environment settings before loading the SDK and suppress its own logger.
• Add a shared settlement helper and extend readiness fixtures to use the prepared observer.

Developer impact

Internal tooling only; no published Hydrogen changes or changeset. No buyer is created here. Startup needs no buyer key, although the SDK constructor can inspect native-provider environment settings for its unused default client.

Risk

Early attachment does not remove Stagehand's frame limitations, including A→B→A nesting. SDK error messages remain unchanged within the wrapper; excluding request/response properties is not secret redaction.

Stack: 12/15. Previous: #4319. Next: #4321.