Add diagnostic logging to AudioManager and use explicit mixer params

[Copilot]

Audio initialization was silently failing with no way to diagnose why. The mixer was using default parameters which behave inconsistently across environments.

Changes

• src/ui/audio.py – init(): Print each SDL driver attempt with success/failure and error details. Use explicit mixer params (frequency=22050, size=-16, channels=2, buffer=512). Separate ImportError from general exceptions for clarity.
• src/ui/audio.py – _load_sounds(): Print per-file load status and summary count (loaded N/7 sounds).
• main.py: Log whether audio init succeeded or fell back to silent mode.
• Tests: 3 new tests covering diagnostic output for disabled, uninitialized, and missing-files cases.

Example output

AudioManager: trying default driver
AudioManager: FAILED with driver 'default': No available audio device
AudioManager: trying driver 'pulseaudio'
AudioManager: SUCCESS with driver 'pulseaudio'
AudioManager: mixer config: (22050, -16, 2)
AudioManager: loading sounds from 'data/sfx'
AudioManager: file not found: data/sfx/fire_abm.wav
...
AudioManager: loaded 0/7 sounds
WARNING: Audio system failed to initialize - game will run silently

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.