Read back temporary LAC files in E2E tests

[audexdev]

Summary

Updates E2E coverage so the test writes the encoded .lac bytes to a temporary file, reads that file back, verifies the bytes match the encoder output, and decodes the reloaded bytes.

Closes #21.

Correctness

• Roundtrip behavior is preserved for supported PCM WAV inputs.
• Codec, predictor, residual, stereo, or format behavior changes have targeted tests.
• Bitstream behavior changes are reflected in docs/format.md.
• Unsupported or out-of-scope behavior is documented or rejected explicitly.

Security

• Malformed input, declared sizes, and allocation behavior were considered.
• Parser/decoder/CLI changes fail safely on invalid input.
• Security-sensitive details are kept out of public comments when private reporting is appropriate.

Fuzzing Impact

• The change does not reduce fuzzability of parser or decoder surfaces.
• New parser/decoder edge cases have regression coverage or a fuzzing follow-up issue.
• Sanitizer behavior was considered for memory-safety-sensitive changes.

Release Impact

• Version, release notes, public API, and documentation impact were considered.
• Compatibility changes are called out clearly.
• No generated files, local fixtures, editor caches, or build artifacts are committed.

CI

• Relevant local build/test commands were run.
• GitHub Actions is expected to pass.

Local verification:

cmake -S . -B build-pr21 -DCMAKE_BUILD_TYPE=Debug -DLAC_BUILD_TESTS=ON -DLAC_TEST_ASSETS_DIR=/tmp/lac-no-assets
cmake --build build-pr21 --parallel
ctest --test-dir build-pr21 --output-on-failure
./build-pr21/lac_cli selftest

Notes

This is intentionally limited to test coverage. It does not add a CLI file-loading path for .lac decode beyond the existing code.