docs: align ID format documentation with implementation (#<N>)

[Copilot]

Documentation referenced _anon_<N> format for generated anonymous subject IDs, but implementation uses #<N>.

Changes

• Specification docs (data-model.md, api.md, research.md): Updated all references from _anon_<N> to #<N> format
• Code cleanup (Serialize.hs): Fixed comment indentation, removed dead commented-out code

Example

Before (docs):

return $ S.Symbol ("_anon_" ++ show n)

After (matches implementation):

return $ S.Symbol ("#" ++ show n)

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