docs(architecture): surface reserved-keyword change in changeset and tests

Address knsv review feedback on #7708:

1. Expand the changeset to call out that align, row, and column are now
   reserved keywords in architecture-beta. Authors using one of these
   as an exact service/group/junction id (e.g. service row(database))
   will get a parse error and need to rename. Prefix-using identifiers
   like rowspan and columnar keep working via langium longer-alt.

2. Add a parametrised unit test asserting that each of the three exact
   reserved-keyword ids is rejected at parse time. This complements the
   existing prefix-collision test (rowspan/columnar resolve clean) by
   pinning the contract for the exact-match case.