Impose limits on facet name length and nesting depth.

[kentonv]

Name length limit: 256 chars
Nesting depth limit: 4 (including root)

We could increase these limits if we needed to but it's hard to imagine anyone needing more than this, in fact I have yet to see a use case for multiple levels of child facets at all.

[ask-bonk]

The facets feature is experimental (the test uses compatibilityFlags = ["experimental"]). Adding limits to an experimental feature is fine and doesn't need a separate compat flag.

After thorough review, I find no actionable issues:

1. Logic correctness - The depth check and name length checks are correct with proper boundary conditions.
2. Safety - No memory safety issues. The recursive getDepth() is bounded by the enforced depth limit.
3. Test coverage - Comprehensive tests for boundary values and all code paths.
4. Backward compatibility - Feature is experimental, so adding limits is acceptable.
5. API design - Error types and messages are clear and appropriate.
6. Code style - Follows KJ conventions properly.

LGTM

github run

[jasnell]

Minor nit: if these are only used directly within actor-state.c++, they likely don't need to be constants on the class and could be moved into an anonymous namespace in actor-state.c++