fix: restore <AssemblyVersion>1.0.0.0</AssemblyVersion> across all src csprojs (C4 post-mortem)

[Chris-Wolfgang]

Restores <AssemblyVersion>1.0.0.0</AssemblyVersion> + adds prerelease-safe <FileVersion> (via regex-strip property function) to every src csproj in this multi-project repo.

Why

The original C4 fanout dropped <AssemblyVersion> on the rationale that the hardcoded 1.0.0 was "stale" relative to released package versions. But that staleness was the correct binding-stability behaviour for libraries that ship a net462 TFM. With SDK-derived AssemblyVersion, every minor/patch release ships a different binding identity, breaking .NET Framework consumers without a binding redirect.

DateTime-Extensions v1.3.0 publicly shipped this regression. This repo has not yet released the regression (the C4 drop is on canonical-unprotected but hasn't reached main). Landing this fix neutralises the footgun across all src csprojs before any merge to main.

Csproj changes (per src project)

  <Version>...</Version>
+ <!-- Pin AssemblyVersion to a fixed binding-stability baseline so consumers
+      do not need to recompile / add binding redirects on every minor/patch
+      bump. Bump only on a deliberate breaking API change. FileVersion +
+      InformationalVersion (the latter auto-derived from <Version>) carry
+      the actual release version. -->
+ <AssemblyVersion>1.0.0.0</AssemblyVersion>
+ <FileVersion>$([System.Text.RegularExpressions.Regex]::Replace("$(Version)", "[-+].*$", "")).0</FileVersion>

The regex-strip property function ensures prerelease <Version> values (e.g., 1.2.3-beta.1) still produce a 4-part numeric AssemblyFileVersion (1.2.3.0) — verified locally on peer repos.

Standard pattern (NodaTime / Newtonsoft / AutoMapper)

Property	Bumps on	Example
AssemblyVersion	breaking change only	1.0.0.0
AssemblyFileVersion	every release	<Version>.0
Version (NuGet)	every release	<Version>

Tracked in the per-repo-release-pilot skill as the corrected Phase 2 step 1.

[Copilot]

Pull request overview

This PR restores stable assembly binding metadata for the two shippable src projects, aligning their .NET Framework-facing assembly identity with the binding-stability rationale described in the PR.

Changes:

• Adds fixed <AssemblyVersion>1.0.0.0</AssemblyVersion> to both source project files.
• Replaces hardcoded <FileVersion>1.0.0</FileVersion> with a prerelease-safe value derived from <Version>.
• Documents the intended AssemblyVersion/FileVersion behavior inline in each project file.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Wolfgang.Etl.TestKit/Wolfgang.Etl.TestKit.csproj	Pins assembly version and derives file version from package version.
src/Wolfgang.Etl.TestKit.Xunit/Wolfgang.Etl.TestKit.Xunit.csproj	Applies the same versioning metadata pattern to the Xunit companion package.