docs: a hosting layer may re-place an arena_exhausted run; the development service does not - #542

…pment service does not

Four docs stated that a hosting layer re-places a run that fails with
arena_exhausted on "the next rung of a trusted ladder, escalation cap 2".
The current development service has no such ladder: it makes one attempt on a
fixed template arena, never retries, and returns the typed failure as
engine_failed. The docs now say a hosting layer MAY re-place the run on a
larger arena, and that the development service does not:
docs/design/arena-bound.md, docs/design/arena-bound-corpus-census.md,
docs/scratch-store.md, and the two matching lines of the memory-management
page. Docs only; no code, no behaviour change.

Removed sentences, named for the doc-reverts gate (both were introduced by
commit 91353de):
- docs/design/arena-bound.md: "A typed `arena_exhausted` re-places the run on
  the next rung of COMPUTE's trusted ladder, with escalation cap 2."
- docs/design/arena-bound-corpus-census.md: "COMPUTE uses the default
  attempt-0 arena and its trusted escalation ladder: a typed `arena_exhausted`
  re-places the run on the next rung, escalation cap 2, never charged to the
  user, with an alert."

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>