tokenize(line) splits a single line into typed tokens:
keyword (cyan) PROGRAM, FUNCTION_BLOCK, IF/THEN/ELSE/END_IF,
FOR/TO/DO, VAR/END_VAR, CASE/OF, AND/OR/XOR/NOT,
TRUE/FALSE, EXTENDS, IMPLEMENTS, etc.
type (magenta) BOOL, INT, DINT, REAL, LREAL, TIME, STRING,
ANY_*, ARRAY, POINTER, REFERENCE, etc.
comment (gray) (* ... *) inline and // line-to-end
string (yellow) 'single' and "double" quoted
text (none) everything else
Identifier matching is case-sensitive uppercase only — matches
typical IEC 61131-3 convention and avoids false positives on lower-
case identifiers like if_x. Multi-line (* ... *) comments are not
joined across lines (out-of-scope for v0.2).
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