audio: add RustChain reward alert cues

[Asti1982]

Summary

• adds a distinct blood-economy reward alert sound set under pk3_build/sound/rewards/rustchain/
• includes six original OGG Vorbis cues: block confirmed, reward mint, wallet credit, chain reorg warning, mining tick burst, and style multiplier lock
• adds deterministic procedural synthesis source plus README and CC0 license

Verification

• python -B -m py_compile tools/audio/generate_rustchain_reward_alerts.py
• python -B -m unittest discover -s tests -p test_reward_alert_sounds.py -v
• python -B -m unittest discover -s tests -v (51 run, 49 passed, 2 optional soundfile skips)
• git diff --check

Bounty context: Scottcjn/rustchain-bounties#293, separate from the already-merged weapon, music, and announcer/stinger sets.