Add incremental edit tests — verify editing one file only regenerates affected contracts

[Skymly]

Problem

IncrementalCacheTests only verifies "same compilation, second run → all stages Cached/Unchanged". This proves the value-equatable model refactoring works, but does not prove the core incremental value of source generators: when one file is edited, only the contracts in that file are regenerated; other contracts remain cached.

This is the primary performance benefit of IIncrementalGenerator in large projects. Without an edit-scenario test, there is no evidence that the incremental pipeline actually works for the real-world case it was designed for.

Current state

SourceGeneratorTestContext.AssertCacheHit runs the generator twice on the same compilation object (not Clone), then checks all tracked stages are Cached/Unchanged. This is a "no-op" cache test.

The Location reference equality limitation (P1-#5) means Clone would force re-execution, so the current test avoids it. But a proper incremental edit test needs to modify a syntax tree and re-run, which is the real scenario.

What's needed

A new test method (or test class) that:

1. Creates a compilation with multiple contracts in separate files (e.g., two [RegisterStrategy] contracts: IShippingStrategy in Shipping.cs and IPaymentStrategy in Payment.cs)
2. Runs the generator once (populates cache)
3. Modifies one file (e.g., adds a new strategy to Shipping.cs, leaves Payment.cs unchanged)
4. Re-runs the generator with the updated compilation
5. Asserts that:
   • The transform stage for the modified file's contract returns Modified (re-executed)
   • The transform stage for the unmodified file's contract returns Cached or Unchanged
   • The Collect stage returns Modified (because one input changed)
   • The Combine stage returns Modified (because Collect changed)

This requires a new helper method in SourceGeneratorTestContext that:

• Accepts an initial set of sources and a modified set of sources
• Runs the generator on the first compilation, then on the second (with Clone + tree replacement)
• Returns the GeneratorDriverRunResult for the second run so the test can inspect tracked step reasons

Affected files

• tests/DesignPatterns.SourceGenerators.Tests/SourceGeneratorTestContext.cs — add RunIncrementalEdit helper
• tests/DesignPatterns.SourceGenerators.Tests/Generators/IncrementalCacheTests.cs — add edit-scenario tests

Test scope

Start with RegisterStrategyGenerator (most representative: dual-branch Collect + Combine). If the pattern works, extend to other generators.

Module

SourceGenerators (tests)