Runs the upgrade host's cascade over a child

[johnnyt]

The upgrade host's cascade test failed a loan and ran
Executions.cascade_cancel/3 over it, but the loan document opens no
subchart, so the answer {:ok, 0} and the :no_linkage read would also
hold if the cascade did nothing. A later bump step that changed the
cascade or the linkage reads would not have been caught.

What changes

• upgrade_host/test/support/loan_with_notice.ex (new, test support): a
  second loan document whose core.subchart names an overdue-notice
  document (which waits for notice.sent), the
  StatifierBlocks.Runtime.DurableSubchart host callbacks that resolve
  it, and a StatifierPersistence.Driver over the host's own store.
• upgrade_host/test/upgrade_host/loan_test.exs: a new test opens such
  a loan through Driver.create/3, finds the notice through
  Linkage.parent_match/1, reads its linkage back through
  Linkage.from_metadata/1 and Driver.parent_link/2 (the parent id,
  the invoke id, the child index, the content hash, the derived child id), fails the
  loan with Executions.fail/3, checks the notice is still active, and
  asserts Executions.cascade_cancel/3 answers {:ok, 1} and leaves the
  notice :cancelled; a second run answers {:ok, 0}. The existing
  no-child test stays as it was, with its comment pointing at the new
  one.
• upgrade_host/README.md: one bullet under "What it holds" for the
  loan with a child execution.

No file under lib/ changes, and the upgrade host's exact pins in
upgrade_host/mix.exs and its mix.lock do not move.

Sabotage

Each mutation was applied, the test run, and the file restored
byte-equal (a dependency mutation recompiled the dependency for the
test env before and after):

• the cascade counted each child without cancelling it
  (statifier_persistence's cancel_counted): red, the notice was still
  active after the cascade;
• Linkage.from_metadata/1 read a key the package never writes: red, the
  notice's linkage read :no_linkage;
• the driver's dispatch answered :pending without starting a child:
  red, the loan had no child.

Gate

Both gates green on the branch: the root mix quality, and from
upgrade_host/ the CI job's steps (mix deps.get --check-locked,
mix compile --warnings-as-errors, mix test against Postgres).

Provenance

Exercising the cascade over a loan with a child, with a README limit
only as the fallback, was decided by the conductor under a standing
consent, 2026-10-10. The bounded attempt fitted inside the test support
with no package change, so the fallback was not taken. No record text
changes in this PR.