Remove Generic-based Flat derivation machinery (#7542)

The internal flat library contained Generic-based Flat class derivation
via GHC.Generics that had a bug: deserializing large enum types (512+
constructors) consumed infinite memory. While not a security risk (UPLC
on-chain code uses only manual Flat instances), the buggy dead code was
distracting. This commit removes the entire Generic derivation machinery.

Changes:

1. Add manual Flat instances to all types that previously relied on
   Generic defaults: Maybe, Either, Complex, [], NonEmpty, tuples (2-7),
   Tree, Filler, PostAligned, PreAligned, SrcSpan, SrcSpans, and all PIR
   types (Recursivity, Strictness, Datatype, Binding, Program, Term).
   Manual encoding is bit-identical to the Generic encoding.

2. Add missing `size` methods to partial Flat instances in
   FlatInstances.hs (Name, Version, VarDecl, TyVarDecl, Program,
   NamedDeBruijn) and Value.hs (K, Quantity, Value) that previously
   relied on the Generic default for `size`.

3. Gut Class.hs: remove ~440 lines of GFlat*/GEncode*/GSize* classes,
   NumConstructors type family, all Generic default method implementations,
   and associated imports/pragmas/re-exports.

4. Remove ENUM_LARGE test infrastructure (E256/E258 types, Flat/Arbitrary
   instances, ~530 lines) and delete Core.hs inspection tests.

5. Fix transitive import breakage: files that got Generic from the
   PlutusCore.Flat re-export now import GHC.Generics directly.
   Remove `hiding (to)` from imports that no longer re-export Generics.

6. Strip Generic-specific doctests from Tutorial.hs.

All existing tests pass (5281 across 4 suites). Manual instances produce
bit-identical encoding verified by round-trip tests and golden files.

Author: Unisay

plutus-core/changelog.d/20260313_161545_yuriy.lazaryev_issue_7542_remove_dead_flat_code.md

@@ -0,0 +1,7 @@
+### Removed
+
+- Generic-based `Flat` class derivation machinery (`GFlat*`, `NumConstructors`, default method implementations) and associated test/doc infrastructure.
+
+### Added
+
+- QuickCheck round-trip property test for PIR Flat serialization.