Add chained tag syntax for nested tag payloads

[fasterthanlime]

Summary

Add chained tag syntax so one tag can wrap exactly one nested tagged value without changing the meaning of ().

Changes

• extend tokenization, parsing, and CST handling for @outer/@inner... syntax
• lower chained tags to nested tag events so facet-styx can deserialize nested newtype enum variants
• emit and round-trip chained tags in the formatter and serializer
• document the syntax in the parser reference and primer, and add parser/serializer/round-trip coverage

Testing

• cargo nextest run -p styx-tokenizer -p styx-parse -p styx-cst -p styx-format -p facet-styx