T06: wire cost into the loop

- InnerLoop definition-of-done gains a cost row: M-D2-CST may no longer
  be recorded uncomputable, and composition must be reported, not only a
  total.
- InnerLoop Step 5 gains the --self-test contract: every tool that
  reports a number exposes one, and it runs before the number does.
  Rationale attached, because the case that motivated it is the one
  review cannot catch — survey and reviewer both verified the same large
  sample and both missed the small one.
- make cost / cost-test / cost-pin on the one command surface; cost-test
  in `make all` and in CI.
- Hub now holds the measured figure for CB-WP-0001: 80.6M in / 323.6k
  out against the 401,100 it previously estimated, low by ~200x. The
  event states plainly that the hub schema cannot represent the 88% of
  cost that is cache, and names `make cost-pin` as the authority.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

workplans/CB-WP-0002-cost-accounting.md

@@ -192,7 +192,7 @@ cleared the bar that CB-WP-0001's AM-12 failed to clear.
 
 ```task
 id: CB-WP-0002-T06
-status: todo
+status: done
 priority: low
 state_hub_task_id: "1412263b-70c1-43e4-957d-1ad6c3203ca9"
 ```