Adopt a consistent error-handling strategy across encode, decode, I/O, and CLI

[audexdev]

Summary

Error reporting differs at every layer of the pipeline — bool (dropping the reason), thrown std::runtime_error, thrown std::invalid_argument, and a status-enum-plus-string. This makes failures hard to surface uniformly and, for the block decoder, discards the reason a malformed stream was rejected.

Affected areas

• src/codec/block/decoder.cpp
• src/codec/lac/decoder.cpp
• src/codec/lac/encoder.cpp
• src/io/wav_io.cpp
• src/main.cpp

Problem details

• Block::Decoder::decode_into returns plain bool on every rejection, discarding which check failed — debug builds rely on LAC_DEBUG_* logging to recover any context.
• LAC::Decoder::decode throws std::runtime_error with "[decode-error] ..." strings (decoder.cpp:26-33).
• LAC::Encoder::encode throws std::invalid_argument (encoder.cpp:221-242).
• read_wav / write_wav return bool (wav_io.cpp).
• The CLI fast decode path returns a FastDecodeStatus enum plus an out-param std::string error (main.cpp:74-78,183-430).

The repo's stated direction is to "reject malformed input cleanly" and add malformed-input regression tests; a bool that loses the reason makes those tests assert only pass/fail and makes diagnostics depend on debug-only logging.

This is related to #7, which asks for consistent decode-time rejection of non-canonical metadata. This issue is the broader design question: one error/result strategy across encode, decode, I/O, and CLI.

Acceptance criteria

• A single, documented error-reporting convention is chosen for the codec (e.g. a small Result/status type carrying a reason, or a consistent exception policy at the public boundary).
• The block decoder can report why a stream was rejected without relying on NDEBUG-gated logging.
• Container and block decode layers report failures consistently (aligns with Reject non-canonical LAC and block metadata during decode #7).
• Malformed-input tests can assert on the specific rejection reason.