table: skip validator registration for no-op producers. Parent: #830

[hectar-glitches]

Summary

This PR gates validator registration on a new needsValidation() bool method on the producerImpl interface:

• fastAppendFiles returns false: appends are commutative and need no conflict checks
• mergeAppendFiles inherits false via embedding
• overwriteFiles returns true: real conflict validation required
• snapshotProducer.commit() skips registration when needsValidation() is false

The no-op validate() methods are kept as belt-and-suspenders.

Behavior is locked with two unit tests asserting that fast-append and merge-append commits leave txn.validators empty.

Parent: #830