decompress: return the skippable header size from ZSTD_frameHeaderSize()

[ilyas-mallah]

What and why

For a skippable frame, ZSTD_getFrameHeader() reports a header size of 8, as the format spec says: a 4-byte Magic_Number and a 4-byte Frame_Size. ZSTD_frameHeaderSize() doesn't check the magic number. It reads the low byte of Frame_Size as a Frame_Header_Descriptor and returns whatever header size that byte would describe, anything from 6 to 18. A skippable frame with 5 bytes of content gives 7.

Internal callers only use the internal version for zstd frames, so only the public function's result changes. The new test sits with the other skippable frame tests in tests/fuzzer.c, fails on dev and passes with this change, and make -C tests test-fuzzer passes.

Checklist

• I searched open pull requests and issues: this problem is not already being addressed.
• Bug fix: it comes with a reproducer or a test that fails without the fix.
• Performance change: numbers from repeated runs are included, with CPU, compiler, flags and data.
• I stated below whether AI or other automated tools were used.

Tools used

AI usage: Claude Code helped with the test runs. I traced the mismatch, made the fix, and validated every change.

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[ceedot-rock]

@Cyan4973 — small API correctness fix: ZSTD_frameHeaderSize() now returns the skippable header size for skippable frames. Clean 10-line change. Anything blocking?