C# nested submodules

[lisandroct]

Description of Changes

This PR adds support for nested C# submodules. Multiple independent instances of the same module assembly can be mounted independently of each other (for example: submodule A can mount submodule C and submodule B can also mount it's own submodule C).

• Route tables, indexes, reducers, procedures, and views to the correct module instance.
• Add ctx.As for passing an instance-selected context to functions that receive context.
• Add instance-aware ctx.From query builders.
• Add context-aware immediate scheduling overloads for reducers and procedures.
• Add nested namespace paths in generated C# client queries (for example ctx.Db.SubmoduleA.SubmoduleC.{...}).

API and ABI breaking changes

No breaking changes.
This PR extends what we added in #5974 but everything stays compatible. Before, if a submodule mounted a submodule, that would throw an error; now it's supported.

Rollback safety impact

n/a

Expected complexity level and risk

4/5. This PR touches a lot of things: it changes module composition, generated accessors, function dispatch, runtime context handling, SQL name generation...

Testing

I added generator diagnostics and snapshot coverage, a nested-module host integration fixture, and client regressions targeting .NET 8 and .NET 10. Coverage includes independent tables and caches, deep namespace paths, dispatch, queries and joins, context selection, transaction commit/rollback, and immediate scheduling.

• Run C# generator tests on .NET 8 and .NET 10.
• Run the namespace_csharp standalone integration tests.
• Run the C# client regression harness against the updated local SDK packages.