docs: align guides with current engine contracts

[magiccodingman]

Summary

• align the version 2 documentation with the current .NET 10 public API and planner behavior
• clarify staged query ordering, pagination, generated-key behavior, and schema validation constraints
• document current predicate semantics, compound-index fallback, compound-key ordering, and planner diagnostics

Validation

• dotnet contract tests: 61 passed
• documentation usage examples compile against the current project
• relative documentation links and whitespace checks pass