[mkitti]

Fixes inconsistent error handling and a few memory-safety problems in src/numcodecs/zstd.pyx. This follows up on the review comments in #848, which suggested these belong in a separate PR against main.

Bugs fixed

• Empty frames could not be decoded. Zstd().decode(Zstd().encode(b"")) raised invalid input data because a content size of 0 was treated as an error. An empty input is still rejected.
• Double free in stream_decompress. If ZSTD_initDStream failed, the DStream was freed inside the nogil block and again in finally.
• free() of an uninitialized pointer. output.dst was freed in finally before it was ever assigned if init failed.
• Unchecked allocations. malloc, ZSTD_createDStream and ZSTD_createCCtx results were never checked; these now raise MemoryError.
• Context leak in compress. ZSTD_createCCtx and ZSTD_CCtx_setParameter ran outside the try, so a parameter error leaked the context.
• Error names formatted from bytes. Messages looked like Zstd decompression error: b"Restored data doesn't match checksum". They are now plain text.
• Lost diagnostics. Failures while determining frame sizes were collapsed into a bare invalid input data; the zstd reason is now appended (the existing invalid input data prefix is kept so current matches still work).

Improvements

• Errors in the streaming loop are recorded inside the nogil block and raised after it, rather than raising from within it.
• Truncated streaming input is detected explicitly instead of relying on an earlier check to avoid looping.
• The streaming output buffer grows geometrically (it grew by a fixed 128 KiB per step, which is quadratic for large outputs), with overflow checks.
• Overflow check when summing frame content sizes across concatenated frames; findTotalContentSize now declares its exception behaviour.
• stream_decompress has a bytes return type; dead try/finally: pass removed.
• Zstd.__repr__ now includes checksum when it is enabled (default output is unchanged).

Testing

New tests cover empty frames, empty input, truncated known/unknown-size frames, text error messages, decoding unknown-size frames into out, repr with checksum, and multi-frame streaming growth. Full suite locally: 645 passed, 37 skipped.

Backport

Once this is reviewed, it should be backported to 0.15.x alongside #848.

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.md
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

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