Add typed D2 features and fluent APIs

[Stephanvs]

What changed

• expands D2Style to the documented style catalog with optional named arguments, typed enums, and range validation
• adds typed shape and connection icons, links, tooltips, dimensions, and styles
• adds typed SQL tables, columns, and constraints
• adds typed UML classes, fields, methods, parameters, and visibility
• adds ordered fluent sequence-diagram helpers
• adds D2DiagramBuilder for mutable fluent construction and immutable snapshots
• makes shape labels optional and adds PascalCase direction aliases while preserving obsolete uppercase aliases
• internalizes the legacy serialization helper
• aligns the UML member hierarchy across netstandard2.0, net8.0, and net10.0
• adds typed examples and broad parser-backed coverage

Validation

• dotnet test: 33/33 passed
• three real d2 validate integration paths pass
• all three package target frameworks build cleanly
• package integration confirms no compatible-framework API mismatch
• documented style ranges and invalid enum values are tested
• git diff --check: passed

Compatibility

This is a major-version API change: the style constructor expands, the legacy helper becomes internal, and uppercase direction fields are deprecated (but preserved).

Stack dependency

This PR targets feature/extensible-model. Merge PR #4, then PR #7, and retarget this PR to main before merging.