Add Inflate::try_clone, a safe inflateCopy

[zone117x]

The C API has had inflateCopy from the start; the Rust Inflate type had no way to copy a decompressor. This adds Inflate::try_clone, which returns a copy at the current point, from which both continue independently. It is what random access into a deflate stream is built on (zlib's zran example, or a state copy every few MiB): copies saved while the stream is read through, decoding resumed later from the nearest one below the wanted position. We use it for a checkpoint index over gzip in an archive manager, so that a member of a .tar.gz opens in milliseconds rather than after a full decode; until now that meant calling inflateCopy through libz-rs-sys from Rust, since the inflate module sits behind __internal-api.

inflate::copy refused a stream that had not run yet (null next_out). The buffer pointers are copied, never read, so that check is gone; the one inconsistent state, input claimed without a buffer, is still rejected. Two unit tests cover a copy mid-stream and one before the first call.

try_clone rather than Clone, since the state allocation can fail and the crate reports MemError everywhere else.