feat(compiler): split frontend — keyword unification

[julia-script]

OpenSpec: split-compiler-frontend

Wave C. Partial — keyword unification done.

Done

• ✅ Moved fn/let/move/pub/return/import into keywordSpellings table
• ✅ Deleted manual per-byte keywordKind block
• ✅ Lexer tests: 54/54 pass

Still needed

• Extract Parser/*.ts sub-modules (Expression, Type, Statement, Declaration, Import)
• internal/ParseState.ts
• Grammar seam merges
• Trivia-skipping peek(n)

Depends on

• feat(compiler): extract shared helpers — ByteClass, Align, Graph, canonicalNaN (partial) #231 (extract-compiler-shared-helpers — ByteClass)

[julia-script]

Superseded by the unified PR.