Admit sandboxed webview downloads through bounded typed leases #535

Parent: #268. Webview epic: #264. Release epic: #227. Host consumers: AppScene #32/#123.

Proven gap

Nested download anchors need browser-shaped sandbox and activation admission plus a typed transfer boundary. Existing behavior falls through legacy save/JSON paths, does not require allow-downloads through every sandboxed owner, and cannot give AppScene immutable generation-stamped source data without also granting destination authority.

Focused implementation

• Require native user activation within five seconds and allow-downloads on every sandboxed iframe ancestor.
• Add a typed download request/lease carrying request ID, origin, document/frame generations, frame owner, target node, sanitized suggested name, MIME, exact/unknown size, and byte/URL/canvas source identity.
• Keep destination choice, overwrite policy, filesystem grants, and large-file streaming in AppScene.
• Support incremental host copying of immutable byte leases; release represents cancellation.
• Cancel queued leases on top-level or owning-frame navigation while preserving taken generations for stale-host rejection.
• Limit the queue to 16 requests, byte sources to 64 MiB per request and 64 MiB aggregate queued payload, names/origins to 4 KiB, MIME to 256 bytes, and URLs to 8 KiB.
• Preserve exact Blob bytes/MIME and retained canvas identity; keep input[type=file] on its existing path.
• Add native nested and file-service regression sources.

Acceptance and deferred gates

• Implementation rebased on WebScene main 987b843076035ae86a6cb4279054ebdef79b0375.
• Focused regression sources authored.
• git diff --check passed.
• Execute native/browser security and lifecycle contracts.
• Implement/qualify AppScene destination UI, durable grants, errors, and large-file streaming.
• Run exact unchanged-webview, performance, memory, package, and three-platform gates.

Execution is deferred under the current implementation-first direction.