Handle explicit sharding in array truncation - #79

Fixes #70.

Explicitly-sharded JAX arrays can reject ordinary slicing when the truncated dimension is not divisible by the mesh axis. Treescope currently hits that failure in _truncate_part_with_slices before the JIT out_shardings constraint can make the final result replicated.

For explicit NamedSharding, convert the first shrinking slice to a gather with an explicitly replicated output on the same mesh. Remaining slices then operate on a replicated intermediate, while non-explicit and older JAX paths keep the existing behavior.

The regression test forces four CPU devices in a subprocess and exercises JAXArrayAdapter.get_array_data_with_truncation with an explicit P("tp", None) sharding, verifying the 7x7 edge data and validity mask.

Local validation:

• reproduced the original ShardingTypeError with four explicit CPU shards
• patched truncation returned the expected 7x7 data and mask
• ordinary non-explicit truncation returned the expected data and mask
• Python syntax compilation passed

The full repository CI toolchain was not run locally because the environment could not fetch the development dependencies. The current Unittests workflow requires maintainer approval.