build: accept in-memory descriptor set bytes

[Yong-yuan-X]

Summary

Build scripts that prepare descriptor bytes in memory currently need to write a temporary file before passing them to buffa-build.

Add Config::descriptor_set_bytes accepting Vec<u8> or &[u8], while preserving the existing path-based descriptor_set API. Owned vectors are accepted without copying, and callers manage Cargo rebuild directives for the sources of the bytes.

Add regression coverage for output parity with file input, file selection, imported descriptors, reflection sidecars, source replacement, and malformed input, along with an unreleased changelog fragment.

Closes #407.

Validation

• All 44 buffa-build unit tests pass.
• The new documentation example compiles.
• Formatting, workspace Clippy, and strict buffa-build Rustdoc checks pass.

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