Route unhandled editor commands to embedders

[yvos]

Summary

• Add a small public command enum for Escape, Tab, and Shift-Tab.
• Add an optional synchronous Bool-returning host callback.
• Route inline previews first, list indentation and outdentation second, and the host exactly once afterward.
• Preserve AppKit fallback when the host declines and use the same fallback contract in raw-source mode.
• Share Tab list handling between command routing and text insertion while preserving maximum-depth consumption.
• Forward top-level Backtab, code-block Tab, and disabled-helper Tab after the engine declines them.

This is the command-routing half of #173.

Validation

• UnhandledCommandTests: 14 tests passed, including two parameterized host-result cases.
• Full suite: 484 tests in 69 suites passed.
• Demo reports host-visible commands and consumes Escape as an example.