Add client action local admission errors

[ogyrec-o]

Summary

Adds explicit local client-action admission error variants.

These variants let the engine reject rapid/conflicting owner prediction requests before they become pending network commands, without abusing TooManyPending or stringly-typed Other errors.

New local admission variants:

• LocalPredictionNoop
• LocalPredictionConflict
• LocalPredictionBacklog { pending, limit }

These are local/engine-side submit errors, not server ClientActionRejectReasons.

Why

freven-engine#348 needs bounded local admission for rapid mixed break/place prediction. The engine should be able to distinguish:

• hard pending queue exhaustion
• predicted edits that would not change the presented/effective client world
• predicted edits that conflict with already in-flight owner prediction
• interactive predicted-edit backlog pressure

ClientActionSubmitError is already #[non_exhaustive], so adding variants preserves the intended SDK evolution boundary.

Validation

• cargo +stable fmt --all
• cargo +stable fmt --all -- --check
• cargo +stable test --locked --workspace
• cargo +stable clippy --locked --workspace --all-targets -- -D warnings
• git --no-pager diff --check

Related:

• freven-engine#348