fix(types): normalize nullable array shapes

[sidux]

Why

Equivalent nullable array-shape spellings could survive as separate union members. Repeated control-flow joins then multiplied those equivalent variants, increasing type size and making workspace diagnostics progressively more expensive.

Summary

• normalize nullable array shapes before union insertion
• flatten and deduplicate equivalent nullable variants during joins
• cover repeated shape merges with a regression test

Validation

• cargo clippy --all-targets -- -D warnings
• cargo test: 14,092 passed; 12 ignored

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