Support full course module transpilation including float_io.occ

[associate-1]

Summary

• Implement Phase 2 features to transpile the complete KRoC course module with float_io.occ (598 parse errors → 0, go vet clean)
• Add RETYPES declarations, transputer intrinsic helpers, array literals, untyped VAL abbreviations, multi-line expression continuation, and CAUSEERROR support
• Add 13 new tests (5 parser, 4 codegen, 4 e2e) covering all new constructs

Lexer

• Bracket [/] NEWLINE suppression via parenDepth
• RETYPES keyword token
• Multi-line expression continuation: isContinuationOp() suppresses NEWLINE/INDENT/DEDENT after binary operators and :=

AST

• ArrayLiteral expression node ([1, 2, 3])
• RetypesDecl statement node (VAL INT X RETYPES X :)

Parser

• Untyped VAL abbreviations (VAL name IS expr :)
• Array literal expressions ([e1, e2, ...])
• RETYPES declarations (single VAL INT X RETYPES X : and array VAL [2]INT X RETYPES X :)

Codegen

• Array literal → []int{...}
• Untyped VAL → var x = expr
• RETYPES with parameter renaming for same-name shadowing (Go can't := redeclare params)
• 7 transputer intrinsic Go helpers (LONGPROD, LONGDIV, LONGSUM, LONGDIFF, NORMALISE, SHIFTRIGHT, SHIFTLEFT)
• CAUSEERROR → panic("CAUSEERROR")
• Go reserved word escaping (byte → _byte, etc.)
• Scoped nested proc signature collection (fixes same-named nested procs)
• Fixed-size array params → slices for Go compatibility
• Abbreviation _ = name suppression in function bodies

Test plan

• go test ./... — all existing + 13 new tests pass
• go vet /tmp/course_out.go — full course.module transpiles cleanly
• go vet on course_nofloat.module — regression check passes

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