feat: integrate schema.lua runtime validation

[mcanouil]

Summary

• Add the shared schema.lua module to _modules/.
• Wire up schema.validate_options() to validate extension options against _schema.yml at runtime.
• Validation logs errors and warnings for invalid configuration (Tier 2 integration: error reporting only, existing option-reading code preserved).

Test plan

• Run quarto render example.qmd and confirm no runtime errors.
• Verify that valid options produce no warnings.
• Test with intentionally invalid options and verify error/warning messages are logged.