Move Bound struct out of SpacetimeDB.Internal to SpacetimeDB and Local out of SpacetiemDB.Runtime

[rekhoff]

Description of Changes

This PR fixes a C# SDK regression where using Bound in index filters could trigger an ambiguous reference compiler error for Local after upgrading to v1.11.2, as reported in #3995.
It also fixes a related warning-spam regression (CS0436) where user projects could see Local type conflicts between generated module code and the SpacetimeDB.Runtime assembly.

• Introduced a public SpacetimeDB.Bound type so users no longer need to import SpacetimeDB.Internal to use bounds in index filters.
• Kept SpacetimeDB.Internal.Bound for compatibility, but added implicit conversions between SpacetimeDB.Internal.Bound and SpacetimeDB.Bound.
• Updated the C# code generator to emit fully-qualified global::SpacetimeDB.Bound in generated index filter signatures, avoiding SpacetimeDB.Internal in public-facing APIs and preventing name collisions (e.g., Local).
• Updated internal runtime bounds helpers (BTreeIndexBounds<...>) to explicitly use SpacetimeDB.Bound when constructing range-scan arguments.
• Updated Codegen snapshot fixtures to match the new generated output (type name + formatting).
• Fixed codegen output for ITableView static abstract member implementations to generate public static methods (required for the generated code to compile).
  It also fixes a related warning-spam regression (CS0436) where user projects could see Local type conflicts between generated module code and the SpacetimeDB.Runtime assembly.

Additional fix (related to the Local reports):

• Removed the runtime assembly’s ownership of SpacetimeDB.Local (introduced more recently than the generated module Local) to prevent CS0436 duplicate-type warnings. Basically, the runtime’s concrete Local/ProcedureTxContext helpers were renamed and made internal so the code generator remains the sole owner of module-level SpacetimeDB.Local.

Regression coverage:

• Added generator regression assertions to ensure generated code does not reference global::SpacetimeDB.Internal.Bound<...> and does reference global::SpacetimeDB.Bound<...>.
• Added a runtime API regression assertion that SpacetimeDB.Bound exists and is public in the runtime reference.
• Added a regression assertion that SpacetimeDB.Runtime does not define codegen-owned types (e.g. SpacetimeDB.Local, ProcedureContext, etc.) to prevent future CS0436 conflicts.
• Added a “simulated downstream user file” compile check ensuring no CS0436 diagnostics occur when user code references SpacetimeDB.Local.

API and ABI breaking changes

None.

• No schema or wire-format changes.
• The changes are limited to C# type exposure / naming and codegen output.
• SpacetimeDB.Internal.Bound remains usable via implicit conversions (backwards compatible for existing code).

Expected complexity level and risk

2 - Low

• Changes are isolated to C# runtime type exposure, codegen type references, and snapshot updates.
• No runtime behavior changes to index scan encoding/decoding; only avoids requiring SpacetimeDB.Internal in user code.

Testing

• Ran:dotnet test crates/bindings-csharp/Codegen.Tests/Codegen.Tests.csproj
• Ran regression tests locally.