Fix empty Blosc buffer round-trips

[fallenmi]

Fixes #831.

Summary

• validate Blosc input before reading frame metadata
• treat a validated zero-length frame as a successful decompression
• require system Blosc 1.16.0+, where blosc_cbuffer_validate was introduced
• add regression coverage and an unreleased fix note

The stricter validation also rejects truncated or size-mismatched buffers, including frames with undeclared trailing bytes.

Validation

• focused Blosc regression/error tests: 15 passed, 1 expected skip
• full local suite: 650 passed, 32 optional-dependency skips
• adversarial empty/non-empty, buffer-type, compressor, shuffle, threading, and malformed-frame matrix: 6,193 checks, 0 errors
• pre-commit run --all-files: passed
• git diff --check: clean

Checklist

• Unit tests and/or doctests in docstrings
• Tests pass locally
• No new or modified user-facing classes or functions
• Changes documented in docs/release.md
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

AI assistance

OpenAI Codex assisted with reproduction, implementation, test planning, validation, and independent review. The change is intentionally opened as a Draft; remote CI and final human review remain pending.