Measure and possibly bound the heap retained by the shared zstd decoder #87

Issue #83 and the EncoderProfileBoundedMemoryUncached profile (#86) cover the zstd encoder only. The decoder is also shared and never closed (sharedZstdDecoder in serialize.go). We have not measured what it keeps live after Decode.

What to measure:

• Live heap after one Decode of a small index and of a large index, with a forced GC before and after.
• The same at GOMAXPROCS 1, 4 and 16, since the decoder may scale its workers with it.
• Whether the 64 MiB window and memory limits change retained heap or only cap it.

Proposed acceptance:

• A benchmark in benchmark_test.go that prints retained MB per GOMAXPROCS.
• A table in docs/ with the real numbers.
• A decision: leave as is, or add an opt-in bounded profile for the decoder, as was done for the encoder.

No code change is proposed until the numbers show a problem.