test: increase code coverage from 77% to 87%

[panbanda]

Summary

• Increase test coverage from 77.42% to 86.69% (~10 percentage point improvement)
• Add 1040 lines of new tests across 10 files
• All 261 tests pass, clippy clean

Test Coverage Details

Files with new tests:

• cli/build.rs: Verbose mode tests for HTML and image export
• cli/validate.rs: Validation stats, human readable output, error display
• exporter/html.rs: Conditional test skip when frontend not built
• model/mod.rs: Duplicate detection, error display, invalid path depth
• model/types.rs: Element trait methods, default type functions, Options
• parser/parser.rs: Empty version/name validation, context errors
• parser/resolver.rs: Flow/deployment validation, nested nodes, suggestions
• parser/writer.rs: Container/component finding and updating
• server/docs.rs: Navigation, markdown rendering, link rewriting
• server/hub.rs: Hub::default() implementation

Coverage breakdown:

Module	Before	After
cli/build.rs	82%	98%
parser/parser.rs	85%	98%
server/hub.rs	93%	100%
model/types.rs	100%	100%
model/mod.rs	99%	99%

Remaining uncovered code:

The remaining ~13% gap to 95% consists primarily of:

• Runtime server code (cli/serve.rs, server/mod.rs) requiring actual server execution
• WebSocket handlers requiring integration testing
• CLI dispatch code parsing command-line arguments
• Async HTTP handlers in server/docs.rs

These would require integration tests rather than unit tests to cover.

Test plan

• All 261 tests pass
• Clippy clean
• Coverage verified with cargo tarpaulin

Generated with Claude Code

[panbanda]

@panbanda Ready for review - coverage increased from 77% to 87% with comprehensive unit tests.