Unify debug/configuration mechanism and remove hidden env coupling from the library

[audexdev]

Summary

Debug and runtime configuration use inconsistent mechanisms: the encoder takes a growing set of boolean flags via constructor/setters, while the decoder reads debug switches from global environment variables; and the library encode/decode entry points silently read process environment variables for thread limits. This couples library behavior to ambient process state and makes the codec hard to drive from tests or as an embedded library.

Affected areas

• src/codec/lac/thread_limit.hpp
• src/utils/logger.hpp
• src/codec/lac/encoder.hpp / src/codec/block/encoder.hpp
• src/codec/block/decoder.cpp

Problem details

• Hidden env coupling inside library calls. resolve_thread_limit calls std::getenv("LAC_THREADS") (thread_limit.hpp:30-33) from deep inside Encoder::encode / Decoder::decode. A library call thus depends on process environment, is not thread-safe against setenv, and has unclear precedence against the explicit --threads / set_thread_count path.
• Two debug mechanisms for one codec. The encoder threads debug state through booleans debug_lpc / debug_zr / debug_partitions / debug_stereo_est (lac/encoder.hpp:34-41, block/encoder.hpp:22-28), each with its own setter, while the decoder reads LAC_DEBUG_ZR / LAC_DEBUG_PART via macros (decoder.cpp:66-67, logger.hpp:18-40).
• Boolean-flag soup. The block/encoder configuration is a pile of independent booleans plus block_index, all set individually — a configuration-object smell that grows with each new debug switch.
• Debug logging writes std::cerr unsynchronized from multi-threaded encode/decode (logger.hpp:49), so concurrent worker output can interleave.

Acceptance criteria

• Thread-limit resolution does not read environment variables from inside the codec; the CLI resolves LAC_THREADS / --threads and passes an explicit value to the library.
• A single configuration mechanism (e.g. one options struct) covers encoder and decoder debug/tuning switches, instead of constructor booleans on one side and env-var macros on the other.
• Documented, deterministic precedence for any remaining environment-driven behavior.
• Concurrent debug logging does not interleave (or is documented as debug-only/unsupported under threading).

[audexdev]

Partially addressed by #34 (merged): the library no longer reads getenv("LAC_THREADS"). resolve_thread_limit() was removed; the encoder/decoder use the explicit thread_count they are given (0 = auto), and the CLI resolves --threads (precedence) / LAC_THREADS once via resolve_cli_thread_count(). CLI behavior is unchanged; README/precedence documented.

Remaining for this issue:

• Unify the debug-config mechanism (encoder boolean flags vs decoder LAC_DEBUG_* env macros).
• Unsynchronized debug logging to std::cerr from worker threads.

These are debug-build-only and overlap #25/#5, so they are left for a follow-up. Keeping this issue open until they land.