Task: Remove stable_arena and close the XML story

Delete stable_arena and its test, then bring the story header up to date and close it.

• stable_arena.hpp, its test and their cmake entries are gone
• The story states what it delivered and is DONE
• The ores.ore build and tests pass

Main added stable_arena in #3584 so a wide-record arena could be a pointer-stable container on MSVC. The trade columns of #3583 removed every arena the portfolio needed, and the generator no longer emits stable_arena. Delete the header, its test and their cmake entries. Then update the story header and close it.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Every number this task claims about a tree, with the command that produced it and the commit it was measured at. A number without both is not a measurement, because a clean-up record describes a tree and trunk moves under a branch: a consumer count, a gate's component count and a suite's size all change with no code changing, and the reader of a bare number cannot tell which tree it came from. Where the raw output matters, put it in an evidence/ file beside the task and name that file in the Command column.

The generator still emits a plain std::vector arena for a recursive schema. The Python tests cover that path.

Manual QA scenarios (scaffolded via compass add test_scenario) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.