Resolve LAC_THREADS in the CLI, not inside the library

[audexdev]

What

Phase 1 of docs/refactor-roadmap.md (#27): stop the library from reading the environment.

The encoder and decoder previously called resolve_thread_limit() inside encode() / decode(), which read getenv("LAC_THREADS"). A library call should not depend on ambient process state, and that read is not safe against a concurrent setenv.

Changes

• Remove resolve_thread_limit() from thread_limit.hpp (keep the pure parse_thread_limit() string parser).
• Block/LAC encoder and decoder now use their explicit thread_count directly (0 = auto) and no longer include thread_limit.hpp.
• main.cpp resolves the thread count once via resolve_cli_thread_count(): --threads wins, otherwise LAC_THREADS, and passes an explicit value to the library (encode + both decode paths).
• README: clarify that LAC_THREADS applies to the lac_cli binary (and CLI subprocess tests); internal codec unit tests set their own thread counts.

Behavior

• Via the CLI: unchanged — --threads=N still overrides LAC_THREADS, both still cap workers.
• Direct library use: no longer reads the environment. Callers pass set_thread_count() (0 = auto). This is the intended point of the change. docs/supported-formats.md already documents --threads / LAC_THREADS as CLI options, which remains accurate.

Deferred

• The broader debug-config unification in Unify debug/configuration mechanism and remove hidden env coupling from the library #27 (encoder boolean flags vs decoder LAC_DEBUG_* env macros, and unsynchronized debug logging) is not in this PR. The LAC_DEBUG_* reads are debug-build-only and tie into Decompose the monolithic block/frame encode functions #25/Make multithreaded encoder startup exception-safe #5; left for a follow-up. Keeping Unify debug/configuration mechanism and remove hidden env coupling from the library #27 open for that.

Verification (local, macOS)

• Clean configure + build succeeds; resolve_thread_limit is gone and src/codec/ contains no getenv.
• ctest: 2/2 suites pass, including the explicit thread-count assertions in run_thread_limit_tests / run_decoder_thread_tests (which use set_thread_count, not the environment).
• lac_cli selftest: passes.

Refs #27