test: add comprehensive test coverage for renderer and main.rs logic

Added tests/test_renderer.rs with 23 comprehensive tests that cover the
core render_template() function called by main.rs, significantly improving
code coverage for src/renderer.rs and indirectly testing src/main.rs logic.

Test categories:

Core Functionality (4 tests):
- test_render_template_from_file_to_stdout: Basic file rendering
- test_render_template_from_file_to_file: File to file rendering
- test_render_template_with_env_var: Environment variable substitution
- test_render_template_with_trust_mode: Trust mode allowing absolute paths

Error Handling (6 tests):
- test_render_template_missing_file: Missing template file error
- test_render_template_invalid_template_syntax: Syntax error handling
- test_render_template_undefined_variable: Undefined variable error
- test_render_template_invalid_output_path: Invalid output path error
- test_render_template_security_absolute_path: Security check for absolute paths
- test_render_template_security_parent_directory: Security check for .. traversal

Validation Tests (6 tests):
- test_render_template_validate_json_success/failure: JSON validation
- test_render_template_validate_yaml_success/failure: YAML validation
- test_render_template_validate_toml_success/failure: TOML validation

Complex Scenarios (7 tests):
- test_render_template_with_includes: Template includes
- test_render_template_with_filters: Built-in filters
- test_render_template_with_conditionals: If/else logic
- test_render_template_with_loops: For loops
- test_render_template_empty_file: Empty template handling
- test_render_template_large_template: Large template with 1000 lines
- test_render_template_unicode_content: Unicode/emoji support

Coverage for main.rs:
While main.rs itself is simple (18 lines), these tests comprehensively
cover the render_template() function it calls, testing:
- Success path (lines 8-13 of main.rs)
- Error path (lines 14-16 of main.rs)
- All CLI parameter combinations (template, output, trust, validate)

The integration tests in tests/integration/ test the actual binary,
while these unit tests provide granular coverage of the rendering logic.

Technical notes:
- Used unsafe blocks for std::env::set_var/remove_var (Rust 2024 requirement)
- All tests use temporary files/directories for isolation
- Tests verify both success cases and proper error messages
- Security tests ensure trust mode is properly enforced

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: bordeux