Add opt-in ~/ asset-path expansion for Razor components (compiler)

.github/instructions/Razor.instructions.md

@@ -40,6 +40,13 @@ their original sub-tree layout
   `solution.GetDocumentIdsWithFilePath(filePath)` then `solution.GetAdditionalDocument(documentId)`.
 - **Remote services**: Place the public stub method (calling `RunServiceAsync`) directly
   above its private implementation method.
+- **Runtime-declared attribute lists**: When the runtime declares a set the compiler must read
+  (e.g. `[EventHandler]`, `[AcceptsAssetPath]`), it applies the attributes to a public type with
+  a well-known name (`EventHandlers`, `AssetPathAttributes`). A `TagHelperProducer` under
+  `Language/TagHelpers/Producers/` keys off that type name (`IsCandidateType`) and emits carrier
+  `TagHelperDescriptor`s whose descriptor-level metadata carries the parsed values. A later
+  optimization pass reads the full discovered set via `ITagHelperFeature.GetTagHelpers()` (not the
+  document's in-scope tag helpers, which are namespace-scoped) and filters by metadata kind.
 - **Visual Studio options**: Register Razor Advanced settings in
   `Microsoft.VisualStudio.RazorExtension\UnifiedSettings\razor.registration.json`, localize
   their UI text in `VSPackage.resx`, read them through `OptionsStorage`, and add remotely consumed

.github/memory/testing/razor.md

@@ -19,3 +19,22 @@ Layer-specific test guidance for Razor tooling/compiler tests under `src/Razor`.
   (Cohosting architecture).
 - Integration tests using `AdditionalSyntaxTrees` for tag helper discovery must
   set `UseTwoPhaseCompilation => true` (see `ComponentDiscoveryIntegrationTest`).
+  Under two-phase compilation the `AdditionalSyntaxTrees` are compiled into a
+  temp assembly and added as a *reference*, so discovery sees those types as
+  coming from a referenced assembly (not source).
+- After Razor compiler tests or their `TestFiles` change, run the complete
+  affected test project. A successful build does not validate embedded
+  baseline resources.
+
+## Baseline (codegen) tests
+
+- `ComponentCodeGenerationTestBase` and similar baseline tests assert generated
+  IR/C#/mappings against files under
+  `src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/TestFiles/IntegrationTests/ComponentCodeGenerationTest/<TestName>/`.
+- Regenerate baselines by building/running the test project with
+  `/p:GenerateBaselines=true` on one CoreCLR target framework, e.g.
+  `dotnet test src\Razor\src\Compiler\Microsoft.AspNetCore.Razor.Language\test\Microsoft.AspNetCore.Razor.Language.UnitTests.csproj --filter FullyQualifiedName~<Name> /p:GenerateBaselines=true`.
+  Tests always "pass" while generating (they overwrite baselines); re-run without
+  the flag to actually validate, and `git diff` the `TestFiles` to review changes.
+  Two-phase tests can also produce `.decl.codegen.cs` / `.decl.mappings.txt`
+  (the declaration half) alongside the implementation and component baselines.