Add academic extension for scientific documents

[gvonness-apolitical]

Summary

• Introduces codex.academic extension for academic and scientific documents
• Adds theorem-like environments (theorem, lemma, definition, proposition, corollary, conjecture, remark, example) with custom variant support
• Adds proof blocks with method annotations (direct, contradiction, induction, cases, etc.)
• Adds multi-part exercises with hints and visibility-controlled solutions (hidden, spoiler, instructor-only)
• Adds exercise sets for grouping problems with shared context
• Adds multi-line equation groups (align, gather, alignat, split) with numbering and custom tags
• Adds algorithm blocks with line-numbered pseudocode and input/output specifications
• Adds cross-reference marks for theorems, equations, and algorithm lines
• Adds configurable numbering system with counter sharing and reset triggers
• Includes comprehensive JSON Schema validation

Files Changed

File	Description
spec/extensions/academic/README.md	Extension specification
schemas/academic.schema.json	JSON Schema for validation
schemas/README.md	Added extension schemas section
scripts/validate-schemas.ts	Added academic schema to validation

Test plan

• Schema compiles and validates correctly (npm test passes)
• All existing tests continue to pass
• Review specification for completeness and consistency