fix(lagoon): warn on after-commit work registered after lagoon:after_commit; correct the OnDatabase example and pivot read guidance (quick-261028-r16)

- flushStatementAfterCommit marks the statement; AfterCommit logs a warning
  when work is buffered on a flushed statement that no longer holds GORM's
  transaction. Log-only: the work still never runs, the write succeeds
- subtests pin the late callback warning, a callback between commit and
  flush, and a reused chained statement staying silent
- README OnDatabase example registers After(gorm:create).Before(commit),
  matching docs/database/transactions.md
- RegisterJoinTable: pivot-ordered reads join the pivot table explicitly;
  a Preload cannot be ordered by a pivot column

docs/database/transactions.md

@@ -84,7 +84,7 @@ return lagoon.OnDatabase(app, func(_ *sql.DB, gdb *gorm.DB) error {
 ```
 
 > [!WARNING]
-> Register such a callback with a `Before("gorm:commit_or_rollback_transaction")` constraint, as above. lagoon runs a single-statement write's after-commit work from its own callback right after that commit step; a callback that GORM sorts after it buffers work that is never run, without an error.
+> Register such a callback with a `Before("gorm:commit_or_rollback_transaction")` constraint, as above. lagoon runs a single-statement write's after-commit work from its own callback right after that commit step; a callback that GORM sorts after it buffers work that is never run. The write still succeeds, and lagoon logs a warning when that happens.
 
 ## Boot-time work that needs the database