Add advanced alt-text demo fixture

[taarikashenafi]

Summary

• add a synthetic advanced fixture for keyword stuffing, contextually inaccurate alt text, and an accurate control
• add a credential-free demo:verify command that emits machine-readable scanner-shaped findings with mocked model verdicts clearly labeled
• add a credential-gated demo:live command that scans only the synthetic fixture with GitHub Models and optional Azure Vision enrichment
• document exact credentials, modes, and demo commands

Technical boundaries

• the five deterministic rules remain enabled and credential-free by default
• keyword stuffing and generated remediation remain part of the opt-in alt-text-quality model rule
• mocked output replaces only the external model response; extraction, rule mapping, and Finding emission use production code
• no GitHub Models or Azure calls were made while preparing this PR

Validation

• npm test (164 tests)
• npm run typecheck
• npm run build
• npm run lint
• npm run format:check
• npm run demo:verify
• verified npm run demo:live refuses to start without a model token