[wieslawsoltes]

Problem

WebScene could request only the ordinary clipboard, so X11 Button2 paste could not select PRIMARY even after AppScene implemented both native selections.

Result

• Adds the ABI-compatible WEBSCENE_HOST_REQUEST_CLIPBOARD_PRIMARY_V1 read flag.
• Admits only known flags on text/plain reads with a live writable text control.
• Issues one PRIMARY read after an uncancelled Button2 gesture.
• Preserves mouseup/auxclick vetoes and Preserve nested anchor activation and external-link disposition #531 anchor/file default actions.
• Keeps ordinary Clipboard API and Ctrl+V requests at zero flags.
• Binds asynchronous completion to request ID, numeric DOM ID, and initiating V8 context, with admission, completion, and pre-dispatch liveness checks.
• Cancels frame/navigation/shutdown work and rejects detached or stale targets.
• Reuses existing 16-pending, 16 MiB, 1,024 aggregate queue, and five-second activation limits.
• Adds focused source contracts and validation documentation.

Automatic PRIMARY publication remains a separate provider capability because browser-workbench Code OSS exposes no product-neutral selected-text signal.

Closes #655.

Validation

git diff --check passed. Under the agreed fast merge policy, no local builds or tests were run.