feat(lint): reject len() in when patterns (E0508) - #27
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`len()` inside `when` computes over the pattern's materialised value (e.g. `[]` for open lists), not the input's, so the result is always a fixed constant. Reject at load time with E0508 and guide users toward `list.MatchN` instead. - Add E0508 diagnostic code with help text - Reject `len` CallExpr in lintWhen walker - Remove `len` from permittedUniverseBuiltins - Remove now-unreachable evaluator test for len-inert-under-subsumption - Add unit test, fixture rule, and scrut integration test - Update AGENTS.md
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len()insidewhencomputes over the pattern's materialised value(e.g.
[]for open lists), not the input's, so the result is always afixed constant. Reject at load time with E0508 and guide users toward
list.MatchNinstead.lenCallExpr in lintWhen walkerlenfrom permittedUniverseBuiltinsExtracted from the @Bind lattice PR; standalone on
main.