docs(bf-34v5a): verify fixture discovery already implemented as standalone module

The fixture_discovery.rs module (516 lines) already provides:
- Standalone module with comprehensive documentation
- Public API: discover_all_fixtures(), discover_fixtures_by_category(), discover_fixtures_flat(), etc.
- Path normalization (symlinks, relative paths, absolute paths)
- Full test coverage (13 unit tests)

All 50 tests pass across the fixture discovery ecosystem:
- fixture_discovery.rs: 13 tests
- cli_invocation_fixtures.rs: 18 tests
- forms_integration.rs: 19 tests

Acceptance criteria verified:
✅ Discovery code is in its own module/file
✅ Provides clear public functions for getting PDF fixtures
✅ Is importable and usable from test code

No code changes required - implementation already complete.

notes/bf-34v5a.md

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+# bf-34v5a: Extract fixture discovery into standalone module
+
+## Summary
+Verified that fixture discovery code is already implemented as a standalone, reusable module at `crates/pdftract-cli/tests/fixture_discovery.rs`.
+
+## Implementation Status
+
+### Module Structure
+- **File**: `crates/pdftract-cli/tests/fixture_discovery.rs` (516 lines)
+- **Module type**: Standalone Rust module with comprehensive documentation
+- **Test coverage**: 13 unit tests, all passing
+
+### Public API Functions
+The module provides the following public functions:
+
+1. **`fixtures_root()`** - Returns the root fixtures directory path
+2. **`discover_all_fixtures()`** - Recursively discovers all PDF files
+3. **`discover_fixtures_by_category(category)`** - Discovers PDFs in a specific category
+4. **`discover_fixtures_flat(dir)`** - Non-recursive discovery in a single directory
+5. **`discover_fixtures_in_dir(dir)`** - Recursive discovery in a specific directory (internal)
+6. **`fixture_categories()`** - Lists all fixture categories present
+7. **`fixture_statistics()`** - Returns summary statistics about fixtures
+
+### Usage in Test Code
+The module is successfully imported and used by:
+- `cli_invocation_fixtures.rs` - Uses `fixtures_root`, `discover_all_fixtures`, `discover_fixtures_by_category`, `discover_fixtures_in_dir`, `fixture_categories`
+- `forms_integration.rs` - Uses `fixtures_root`, `discover_fixtures_by_category`, `discover_fixtures_flat`
+
+### Key Features
+- Path normalization (handles symlinks, relative paths, absolute paths)
+- Recursive and non-recursive discovery modes
+- Category-based filtering
+- Sorted results for deterministic testing
+- Comprehensive test coverage (13 tests)
+
+## Test Results
+All 50 tests passed across the fixture discovery ecosystem:
+- `fixture_discovery.rs`: 13 tests passed
+- `cli_invocation_fixtures.rs`: 18 tests passed
+- `forms_integration.rs`: 19 tests passed
+
+## Acceptance Criteria Verification
+
+### ✅ Discovery code is in its own module/file
+**Status**: COMPLETE
+- File exists at `crates/pdftract-cli/tests/fixture_discovery.rs`
+- Well-documented with module-level and function-level docs
+- Clean, modular code structure
+
+### ✅ Provides a clear public function for getting PDF fixtures
+**Status**: COMPLETE
+- Multiple public functions for different discovery needs
+- Clear function names: `discover_all_fixtures()`, `discover_fixtures_by_category()`, etc.
+- Comprehensive documentation with examples
+- Return types are clearly documented (`Vec<PathBuf>`, `FixtureStats`, etc.)
+
+### ✅ Is importable and usable from test code
+**Status**: COMPLETE
+- Successfully imported via `mod fixture_discovery;` in test files
+- Functions imported via `use fixture_discovery::{...}` statements
+- No compilation errors or import issues
+- Used across multiple test files
+
+## Verification Steps Performed
+1. Reviewed `fixture_discovery.rs` implementation
+2. Verified public API functions
+3. Checked imports in `cli_invocation_fixtures.rs` and `forms_integration.rs`
+4. Ran full test suite: 50/50 tests passed
+5. Verified module documentation and examples
+
+## Conclusion
+The fixture discovery code is already fully extracted into a standalone, reusable module that meets all acceptance criteria. The module provides:
+- Clean separation of concerns
+- Comprehensive public API
+- Full test coverage
+- Clear documentation
+- Reusable patterns for test code
+
+No code changes were required - the implementation was already complete.
+
+## Files Verified
+- `crates/pdftract-cli/tests/fixture_discovery.rs`
+- `crates/pdftract-cli/tests/cli_invocation_fixtures.rs`
+- `crates/pdftract-cli/tests/forms_integration.rs`
+
+## Test Commands Run
+```bash
+cargo test --test fixture_discovery --test cli_invocation_fixtures --test forms_integration
+```
+
+**Result**: 50/50 tests passed in 0.29s