Unskip ExtensionIndexerTests once Roslyn admits indexers in extension blocks

[danfma]

Context

#156 Stage 4 shipped the lowering for C# 14 extension indexers (item$get(self, index) / item$set(self, index, value), custom [IndexerName] support, receiver-once via IrLetExpression). Commit 7bfe7dd.

The 7 tests in tests/Metano.Tests/ExtensionIndexerTests.cs are [Skip]-gated because Roslyn 5.3.0 (.NET 10) rejects indexers inside extension(R) { … } blocks with:

error CS9282: this member is not allowed in an extension block

Even with LangVersion=preview. Verified against the SDK pinned in global.json.

Action when Roslyn unblocks

1. Drop the [Skip(...)] markers in ExtensionIndexerTests.
2. Run the suite. All 7 should pass without code changes — lowering paths are already wired (IrModuleFunctionExtractor.ConvertExtensionIndexerGetter + …Setter, IrExpressionExtractor.BuildExtensionIndexerGetCall + BuildExtensionIndexerAssignment + BuildExtensionIndexerIncrement).
3. Regenerate any sample that exercises indexers; verify generated TS.
4. Close this issue.

References

• Plan unified lowering for C# 14 extension members #156 (Stage 4 was the indexer stage).
• Commit 7bfe7dd — forward-compat scaffolding + skipped tests.
• Roslyn issue: https://github.com/dotnet/roslyn/issues — file a bug / track the LDM thread (TODO if not already filed).

Decision recorded

Forward-compat merge chosen over hold-until-Roslyn so the lowering work doesn't bit-rot — sunk implementation cost is preserved, integration deltas show up the moment the syntax is admitted.