feat: add per-launch Emby endpoint override for LAN soak A/B

[tcconnally]

Summary

• Add HYPERWALL_SERVER_URL for a per-launch Emby endpoint override without mutating config.ini.
• Use it to isolate the observed public Cloudflare delivery failures from Hyperwall rendering/client behavior.
• Add pure config regression coverage and README documentation.

Grounded investigation

PR #54's M5 soak fixed the UI stalls. Greg-side evidence shows Emby can transcode demon_mika_compilation_1 at ~9.8x, while the client uses mb.perseus.observer and reports TLS I/O, repeated range reconnects/client disconnects, and cache starvation. This PR enables a controlled LAN endpoint A/B against http://10.168.168.29:8096 without changing saved credentials or normal endpoint settings.

Validation

• python3 tests/run_all.py — PASS
• git diff --check — PASS

Test command on M5

HYPERWALL_SERVER_URL=http://10.168.168.29:8096 ./soak_wall.sh 5