[Syntax]: Adopt gamecoe's naming convention

[nircoe]

soundcoe's public and internal API used a mixed camelCase/PascalCase naming convention.
This PR renames every type, function, member, and local variable to snake_case, matching gamecoe's own convention.

• Rename core types and enums (sound_state, sound_priority, vec3, the audio-format/property enums) to snake_case
• Rename resource classes (audio_data, sound_buffer, sound_source, resource_manager) to snake_case
• Rename the sound_manager class and its public playback API to snake_case
• Rename math utility functions to snake_case
• Rename the soundcoe.hpp public API free functions and init_config fields to snake_case
• Rename the internal detail namespace to internal
• Update README, ARCHITECTURE, CONTRIBUTING and CLAUDE.md to document the new convention, including a trailing-underscore rule for parameters that collide with their own type name