refactor(driver-sql): move the upsert conflict-target path out of sql-driver.ts into sql-driver-upsert-conflict-target.ts (cut 14)

Move-only. assertConflictTargetHonoured, assertConflictTargetBackedWithoutClause,
introspectKeyIndexes, upsertTenantGuard and upsertConflictRefusal move their
bodies into the new seam module behind UpsertConflictTargetHost; the class keeps
every method as a stub with its docblock and signature. indexCoversConflictTarget,
unbackedConflictTargetCause and refuseAmbiguousConflictTarget move with them;
refuseUnbackedConflictTarget, refuseUpsertConflictOutsideWrittenOrganization and
ConflictTargetVerdict gain export in place.

Co-authored-by: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Cyh2bm4otUm5Vnb5Zw2KDT

Author: claude