Cardano Updates

Currently using ghc-9.14.1 (first 9.14 release).

Remove unnecessary language extensions and GHC options from BuiltinUnit test:
- Removed: BlockArguments, TypeApplications (unused)
- Removed: All -fno-* optimization flags (not needed for test)
- Removed: -ddump-simpl-iterations (debugging only)
- Kept: DataKinds (required by compile TH splice)
- Kept: NoImplicitPrelude, Strict, TemplateHaskell (essential)
- Kept: Plugin options (required for test functionality)

Simplified pragmas from 19 lines to 6 lines. Error messages now show
fully qualified type names (PlutusTx.Builtins.Internal.BuiltinUnit)
which are more informative than unqualified names.

Regenerate golden files affected by fourmolu formatting changes to
line numbers in test code.

Add comprehensive documentation for "Cannot construct a value of type"
errors caused by where-binding stage violations. Documentation includes:
- Explanation of GHC simplification behavior
- Three workarounds (top-level, lazy bindings, inside quotation)
- Code examples for each workaround
- Guidance on variable placement in compile quotations

Add BuiltinUnit test suite with golden files to verify improved error
messages for where-binding stage violations. Tests cover:
- error () in where-binding
- unitval reference in where-binding
- locally defined constructor
- toOpaque () usage

Tests use defer-errors plugin option and goldenUPlc to capture error
messages in golden files for regression testing.

When using where-bindings that reference builtin types within compile
quotations, GHC simplification can generate pattern matches on builtin
constructors that the plugin cannot compile. This results in the cryptic
error "Cannot construct a value of type: BuiltinUnit".

This commit improves the error message to:
- Explain that it's a stage violation in Plinth compilation
- List where variables must be defined (top-level or inside quotation)
- Provide common causes (where-clause functions, local variables)
- Suggest how to fix it (move to top-level)

The improved error detection uses isBuiltinTypeCon to identify builtin
types and stageViolationError to generate helpful error messages.

Add intelligent detection of stage violations when using PlutusTx builtin types.

The previous error message "Cannot construct a value of type: BuiltinUnit" was
cryptic and unhelpful for users encountering stage violations. This change
introduces:

- Template Haskell-based detection of builtin types from PlutusTx.Builtins.Internal
- Module-aware type checking to avoid false positives
- Comprehensive error messages explaining stage violations and how to fix them
- Clear guidance on moving functions to top-level scope

The improved error message explains the root cause (stage violation), provides
actionable solutions (move to top-level, check variable scope), and maintains
the original technical context for advanced debugging.

This addresses the confusing error reported in plutus-private#1626 while
maintaining full compatibility with existing functionality.

Document investigation into disabling mkSimplPass for better error messages.

This investigation explored whether removing the GHC simplifier pass could
improve error messages for stage violations. While disabling the pass does
provide clearer errors, it breaks legitimate plugin functionality that depends
on pre-inlining.

The investigation concludes that targeted error message improvements (like the
stage violation detection) are more practical than removing core functionality.
These notes preserve the analysis for future reference and provide context for
the chosen approach.