fix(cimnotebook): stop highlighting bare "<" operator as IRI in IntelliJ (GH-52)

[spah-soptim]

• The hand-written lexer treated every "<" as the start of an IRI reference and consumed to ">" or end of line, so comparisons like FILTER(?a < ?b) colored the rest of the line as an IRI.
• "<" now only opens an IRI token when a closing ">" follows with no character that IRIREF forbids in between (whitespace/controls, <"{}|^`), matching the TextMate rule the VS Code extension already uses; otherwise it lexes as the less-than operator.
• Add JUnit 5 test wiring to the Gradle build and lexer regression tests.

Closes #52