Enforce sight-required spell targeting

[dajiaohuang]

Summary

• share one authoritative visibility predicate between attacks and spells
• reject sight-required spell targets when the caster is blinded or excluded by an explicit visibility ACL
• preserve explicit visibility of hidden/invisible targets and non-sight spell behavior
• prove rejected public casts consume no action, slot, randomness, concentration, state, or revision

Verification

• full domain tests
• full structured-spell MCP test file
• attack-visibility MCP tests
• independent read-only logic review: no findings
• post-main-merge focused domain/MCP tests
• Ruff and git diff --check

Closes #102