AGR: Detect epsilon-reachable cycles in grammar compiler

[curtisman]

Summary

• Epsilon-reachable cycle detection in grammarCompiler.ts: adds compiling and nullable fields to DefinitionRecord to track in-progress rule compilation and whether a rule alternative can match the empty string (ε). During compilation, the set of rule names entered since the last mandatory token was consumed (epsilonReachable) is threaded through createNamedGrammarRules / createGrammarRules / createGrammarRule. A compile-time error is raised when a rule reference is encountered whose name is already in that set, meaning the grammar would loop infinitely at match time.
• New tests (grammarCompileError.spec.ts): 13 error cases (direct self-reference, mutual cycles, optional parts before back-references, Kleene-star groups, multiple independent cycles, etc.) and 10 valid-grammar cases confirming that non-epsilon recursion (mandatory literal before back-reference, Kleene-plus, number/wildcard variables, etc.) is accepted without error.
• Matcher tests (grammarMatcher.spec.ts): 2 new tests verifying that right-recursive rules (which are valid) correctly match multi-token input at runtime.
• Grammar store tests (grammarStore.spec.ts): minor updates to align with the revised compiler behaviour.

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