PlantUML Rendering Improvements

[alexanderlinne]

The slice exporter wrote every package by its last segment only. PlantUML identifies a package by that bare name, so two packages called Domain under Orders and Billing were drawn as one, and a package arrow (-> Domain) ended in whichever of them PlantUML registered first. This PR first emits the slice packages as a single tree and then identifies every package by its full path. Two test-only commits pin the output before each production change, so every change to the generated diagrams shows up as a snapshot diff. See the commit messages for details.

• Pin PlantUML file builder output with snapshots (d9f30c4), test only. Replaces the expected diagrams assembled from string literals with Verify snapshots, and pins how hand-built slices render today: shared parent packages, a package-only slice with a colour, flat slices next to nested ones, C4 boundaries, and a nested slice with a hyperlink. Three of the pinned outputs are rejected by PlantUML:
  • NestedSliceWithHyperlinkTest is fixed by the next commit.
  • BuildUmlByDependenciesWithObjectsWithNoDependenciesTest and SpecialCharactersInComponentNamesTest declare a class and then draw component arrows ([a] --|> [b]). This already happens on main, whose tests asserted the same strings. This PR doesn't address it.
• Emit slice packages as a single tree (4f575b3), production change. Every shared parent package is now opened once instead of once per slice, and contents are indented by nesting level. The hyperlink now goes in front of the colour instead of after the closing braces. Of the 14 snapshots whose source changed, 13 render exactly as before. The exception is the nested slice with a hyperlink, which PlantUML rejected until now:

• Pin output for same-named packages under different parents (cf80f5b), test only. Adds a SameNamedSubnamespaces fixture in which Orders and Billing both contain Domain.Model and Web depends on Billing.Domain, and pins the broken output in plain and C4 style. The "Before" column below is what this commit pins: both Model components end up in one Domain, Orders is left as an empty package, and the extension arrow runs between the two Models inside the merged package.
• Identify slice packages by their full path (48e2421), production change. Packages keep their own segment as label but are declared with their full path as alias (package "Domain" as App.Billing.Domain), C4 boundary IDs use the same path, and package arrows point at it. Both Domain packages now stay apart:

The C4-style builder snapshot (C4StyleSameNamedBoundariesUnderDifferentParentsTest) is fixed in the same way.

One side effect is visible: once its alias and label differ, PlantUML shows an empty package's alias above its label. Empty packages therefore get a blank placeholder label inside them (label " " as A.Y.__empty__), so they are drawn as a regular package box instead of a flat tab. C4 boundaries don't show their ID and need no placeholder.

This affects the empty packages in the focus-on diagrams. Graphviz now routes the arrow into Slice1 around the outside of the diagram; the diagram still has the same packages, components and dependencies:

Apart from these, the existing diagrams render exactly as before.

How the images were made

Each .verified.txt snapshot was rendered at the commit before and after the change, with PlantUML 1.2026.8 and Graphviz. The remote !include of C4-PlantUML was replaced by PlantUML's bundled copy (!include <C4/C4_Container>). "Renders exactly as before" means the SVGs are identical once the attributes that only encode the diagram source (embedded source, source line numbers, qualified names and element IDs) are removed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.93103% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.33%. Comparing base (61dfcf2) to head (48e2421).

Files with missing lines	Patch %	Lines
...rchUnitNET/Domain/PlantUml/Export/PlantUmlSlice.cs	91.42%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #499      +/-   ##
==========================================
+ Coverage   86.31%   86.33%   +0.02%     
==========================================
  Files         259      260       +1     
  Lines       12450    12496      +46     
  Branches     1210     1216       +6     
==========================================
+ Hits        10746    10789      +43     
- Misses       1370     1372       +2     
- Partials      334      335       +1     

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