Address dangerous GPlate default in Plated typeclass

[Copilot]

The Plated typeclass's default implementation using GPlate silently ignores fields without Generic instances, causing incomplete traversals with no compiler warning.

Changes

• Made Constraint loc instance explicit - The only production instance relying on the unsafe default now has manual pattern matching that explicitly handles all constructors
• Enhanced typeclass documentation - Added warnings with concrete example of silent failure case, clarified Generic refers to GHC.Generics.Generic
• Documented intentional GPlate usage in AST types - Added comments explaining the 4 fallback instances in DataPlated.hs are intentional and type-safe by design

Example of the danger

-- Before: silently skips Baz field during traversal
data Foo = Foo Bar Baz deriving Generic
data Bar = Bar Foo deriving Generic  -- OK: has Generic
data Baz = Baz Foo                   -- PROBLEM: no Generic
instance Plated Foo  -- uses GPlate default, won't traverse Baz!

-- After: explicit implementation or strong warning
instance Plated Foo where
  plate f (Foo bar baz) = Foo <$> f bar <*> pure baz  -- intentional

All 9 Plated instances in the codebase are now either explicit or documented as intentionally using GPlate.

Original prompt

Work on TODO: this is a dangerous function if any component doesn't have Generic instance (from src/Prelude.hs)

Created from VS Code via the GitHub Pull Request extension.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.