[Web] Give callers their own reference to cached shape tuples

[akaashrp]

makeShapeTuple returns the object the shape cache owns. The cache is a 256-entry LRU that disposes what it evicts, so once a prefill touches more than 256 distinct shapes a caller can hold a tuple that has already been freed. WebLLM hits this on a Radeon iGPU as Object has already been disposed followed by a device hang (mlc-ai/web-llm#844). #20130 kept evicted tuples alive until teardown, whereas here the tuple is owned by the caller.

1. makeShapeTuple takes a reference on the cached handle and returns a new object attached to the current scope, so eviction only drops the cache's reference. Like every other function that returns a TVM object, it now requires an open scope.
2. setDeviceLostAutoDispose(false) lets an owner that already disposes the instance itself keep it from disposing on the device-lost promise

A cache hit costs 15 to 30 ns more per call. Tests cover eviction while a caller holds the tuple and deferred disposal after device loss.