docs: align identity format spec with implementation (#<N>)

[Copilot]

The specification documented identity format as _anon_<N> but the implementation in Gram.Transform.generateId uses #<N>. Updated specs to match implementation per reviewer feedback.

Changes

• data-model.md: Updated format from _anon_<N> to #<N> in identity field description and generation rules
• contracts/api.md: Updated fromGram docstring and invariant examples
• research.md: Updated all references including code examples and round-trip examples

-- Implementation format (unchanged)
generateId = do
  i <- get
  put (i + 1)
  return $ S.Symbol ("#" ++ show i)  -- produces #1, #2, etc.

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