Add zero-key prerecorded demo mode

[LucaXTech]

Summary

Adds a zero-key prerecorded demonstration mode for both MAGI and BioAudit.

MAGI

• python -m magi.web --demo
• prerecorded EEG subject-leakage and imbalanced-classification cases
• visible disclosure that no external API calls are made
• static demo fixtures clearly distinguished from real provider telemetry
• no simulated token or latency measurements presented as recorded evidence
• demo runs are not persisted to normal run history

BioAudit

• python -m bioaudit.web --demo
• structured prerecorded methodology audits
• critical/moderate findings with evidence, rationale, repair and verification
• no API keys or provider calls required

Reliability

• adds automated demo-mode tests
• verifies demo workers cannot touch real provider paths
• full test suite: 32 passed

Closes #1