crosscluster/logical: transactional LDR partial unique index predicate transitions defeat the unchanged-skip

[jeffswenson]

Describe the problem

The UC column set is built from key columns only; an UPDATE that keeps the key
columns but moves the row into/out of the index predicate acquires/releases the
unique value with no lock emitted. Partial unique indexes pass txn-mode creation
checks (checkUniqueIndexesMatch compares key columns, not predicates) and
tree.IsAllowedLDRSchemaChange even allows creating one mid-stream (which the
once-at-startup NewLockSynthesizer never sees).

Impact

Missed conflicts on partial unique indexes → duplicate-key errors / divergence.

Code references

• txnlock/schema.go:81-91, schema.go:204-213

Suggested fix

Disable the equality skip for partial unique indexes, or reject them in txn-mode
validation and close the mid-stream loophole.

Epic CRDB-65552

Jira issue: CRDB-65609

[blathers-crl]

cc @cockroachdb/cdc

[blathers-crl]

Hi @jeffswenson, please add a branch-* label to identify the earliest affected branch for this C-bug

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}