Remove dead code and empty placeholder files; move selftest out of the CLI binary

[audexdev]

Summary

Several dead or placeholder artifacts remain in the tree: an unused (and non-deterministic) Rice helper, a stored-but-ignored constructor parameter, empty .cpp/.hpp placeholder files, and a correctness self-test compiled into the production CLI binary. These mislead readers about what is load-bearing.

Affected areas

• src/codec/rice/rice.cpp / src/codec/rice/rice.hpp
• src/codec/block/encoder.hpp / src/codec/block/encoder.cpp
• src/utils/logger.cpp, src/utils/endian.hpp, src/codec/bitstream/bit_reader.cpp
• src/main.cpp

Problem details

• Rice::compute_k is dead code. It is declared (rice.hpp:39) and defined (rice.cpp:51-68) but never called from src/ (only the header/definition exist; tests do not use it). It also uses a double-based estimate (1u<<k) < (uint32_t)(mean+0.5) that differs from the live integer adapt_k, so it is both unused and a cross-compiler non-determinism trap if revived.
• Block::Encoder::order member is effectively dead. The constructor takes and stores order (encoder.hpp:22, encoder.cpp:17), but encode recomputes the order from candidates and never reads this->order; LAC::Encoder passes 12, which is ignored. A parameter that looks load-bearing but is not is a maintenance trap.
• Empty placeholder files. utils/logger.cpp (0 bytes), utils/endian.hpp (0 bytes), and codec/bitstream/bit_reader.cpp (1 line) imply implementations that were never written.
• Self-test embedded in the production binary. lac_cli selftest (main.cpp:794-867) is a correctness round-trip test living in the CLI, not the test suite, and it does not cover per-block stereo (mode 2) or mono.

Acceptance criteria

• Rice::compute_k is removed (or, if a use is intended, documented and made consistent with adapt_k).
• The unused Block::Encoder order parameter/member is removed or actually used.
• Empty placeholder files are removed, or filled with their intended content and a reason to exist.
• The self-test is moved into the test suite (or kept but expanded to cover per-block stereo and mono, with a documented rationale for shipping it in the CLI).

[audexdev]

Partially addressed by #33 (merged): removed Rice::compute_k, deleted the empty placeholder files (bit_reader.cpp, logger.cpp, endian.hpp) and dropped them from CMake, and expanded lac_cli selftest to cover mono and per-block/auto stereo.

Remaining: the dead order parameter on Block::Encoder / LAC::Encoder. Deferred to #25 (decompose encode) because removing it shifts arguments across both constructors, main.cpp, and ~15 call sites in a way that compiles silently. Keeping this issue open until that part lands.