Enforce literal buffer capacity contract

[KSAGlory]

Summary

ZSTD_compressSequencesAndLiterals() documents that litCapacity must be at least 8 bytes larger than litSize, but the implementation only rejected capacities smaller than litSize.

This change enforces the full documented margin using an overflow-safe subtraction check. The regression coverage verifies that litSize + 7 is rejected with ZSTD_error_workSpace_tooSmall and that the exact litSize + 8 boundary succeeds.

Fixes #4779.

Testing

• make -C tests fuzzer -j2
• tests/fuzzer -v -i0 (347 unit checks passed)
• make check -j2 (all tests completed successfully)
• git diff --check