feat: complete code generation architecture for assert/require packages

[fredbi]

Inverts the generation model: internal/assertions is now the single source
of truth, with assert/ and require/ packages fully generated including all
variants (format, forward, tests, examples).

Architecture:

• Scanner: AST/types analysis extracts functions, signatures, and test examples
• Generator: Template-based generation of 76 functions × 8 variants = 608 functions
• Model: Structured representation bridging scanner and generator

Key improvements:

• Example-driven test generation from godoc "Examples:" sections
• Achieves ~100% coverage in generated packages (99.5% - 0.5% gap from helpers)
• Modern Go 1.23 iterator-based table-driven tests throughout
• Domain-organized source (boolean, collection, compare, equal, error, etc.)
• Function type detection for proper generation of type/var function signatures
• Comprehensive documentation (MAINTAINERS.md, CLAUDE.md)
• Codegen smoke tests (24.4% coverage)

Generated packages:

• assert/require: assertions, format variants, forward methods
• All with corresponding tests and runnable examples
• Helper types and interfaces

This replaces the previous semi-hand-written/semi-generated approach where adding a single
assertion required manually updating 6+ files that already had thousands of lines.

Now: write once in internal/assertions/ in small focused source files, run go generate, done.

Trade-off: the existing code generator has been rewritten entirely.
The new one is more complex, but also more readable.
This added (mostly stable) complexity should be outweighted by the
simplification of the assertion development workflow.

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[Copilot]

Pull request overview

This PR implements a comprehensive code generation architecture that inverts the previous model, making internal/assertions the single source of truth for generating both assert and require packages with all their variants. The system now generates 608 functions (76 base functions × 8 variants) with corresponding tests and examples, achieving ~100% test coverage in generated packages.

Key changes:

• New scanner/generator architecture replaces previous semi-manual approach
• Example-driven test generation from godoc "Examples:" sections
• Modern Go 1.23 iterator-based table-driven tests throughout
• Comprehensive documentation added (MAINTAINERS.md, CLAUDE.md)

Reviewed changes

Copilot reviewed 67 out of 161 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
doc.go	Updated package documentation to reflect fork differences and code generation approach
codegen/main.go	New entry point for code generator with configuration flags
codegen/main_test.go	Smoke tests for code generation execution
codegen/internal/scanner/*.go	AST/types analysis for extracting functions and metadata
codegen/internal/generator/*.go	Template-based generation of assertion packages
codegen/internal/model/model.go	Data structures bridging scanner and generator
codegen/internal/generator/templates/*.gotmpl	Templates for generating assert/require variants
assert/*.go	Generated assertion package files
.codecov.yml	Updated to ignore new codegen directory

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[codecov]

Codecov Report

❌ Patch coverage is 99.06542% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.78%. Comparing base (b5da3fc) to head (e378d7d).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
assert/assert_helpers.go	0.00%	10 Missing ⚠️
assert/enable/yaml/enable_yaml.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master       #8       +/-   ##
===========================================
+ Coverage   53.82%   95.78%   +41.96%     
===========================================
  Files          18       37       +19     
  Lines        4964     5627      +663     
===========================================
+ Hits         2672     5390     +2718     
+ Misses       2227      166     -2061     
- Partials       65       71        +6     

Flag	Coverage Δ
oldstable-macos-latest	95.78% <99.06%> (+41.96%)	⬆️
oldstable-ubuntu-latest	95.78% <99.06%> (+41.96%)	⬆️
oldstable-windows-latest	95.78% <99.06%> (+41.96%)	⬆️
stable-macos-latest	95.78% <99.06%> (+41.96%)	⬆️
stable-ubuntu-latest	95.78% <99.06%> (+41.96%)	⬆️
stable-windows-latest	95.78% <99.06%> (+41.96%)	⬆️

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[fredbi]

fixes #6