fix(install): add rust, cpp, csharp to legacy language alias map (#747)

* fix(install): add rust, cpp, csharp to legacy language alias map

The legacy installer compatibility layer in install-manifests.js was
missing entries for rust, cpp, and csharp — languages that have
rules/ directories and (for rust/cpp) install-components.json entries.

Running `./install.sh rust` fails with "Unknown legacy language: rust"
because LEGACY_LANGUAGE_ALIAS_TO_CANONICAL and
LEGACY_LANGUAGE_EXTRA_MODULE_IDS didn't include these languages.

Fixes the issue reported in #694 by @mpiton.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

* fix(install): complete csharp legacy support and add resolution tests

- Add lang:csharp component to install-components.json with
  framework-language module (matching cpp/rust pattern)
- Update csharp mapping in LEGACY_LANGUAGE_EXTRA_MODULE_IDS from
  empty array to ['framework-language']
- Add end-to-end resolution tests for rust, cpp, and csharp verifying
  framework-language module is included in resolved moduleIds

Addresses review feedback from Copilot, Greptile, CodeRabbit, and Cubic.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Happy <yesreply@happy.engineering>

manifests/install-components.json

@@ -210,6 +210,14 @@
         "framework-language"
       ]
     },
+    {
+      "id": "lang:csharp",
+      "family": "language",
+      "description": "C# coding standards and patterns guidance. Currently resolves through the shared framework-language module.",
+      "modules": [
+        "framework-language"
+      ]
+    },
     {
       "id": "framework:laravel",
       "family": "framework",