Partial follow-up to #1670.
Six existing read-only diagnostics endpoints lacked usable OpenAPI
descriptions: process I/O, SQLite metrics, ingestor write counters, and
fleet/node/observer clock skew. This documents their parameters,
empty/error behavior and response root shapes. The known-gap list falls
from 24 to 18.
Descriptions preserve current behavior, including unknown/empty area
fallback, the PacketStore cache ratio, stale write-counter snapshots,
and the single-node 404. Schemas remain minimal; the existing generator
still declares only the generic 200 response, with 404 explained in
prose.
No runtime handler, authentication, configuration or customizer changes.
No added generic response maps.
Validation: regression tests exercise the served spec and real router,
including missing/malformed data, area selection and fallback, and
calibration responses. Full server and ingestor tests pass locally;
post-push Chromium verifies the served spec, six routes and Swagger
rendering. Three independent reviews passed.
Test-only RED `218a3bf3` produced the [six expected documentation
failures](https://github.com/Kpa-clawbot/CoreScope/actions/runs/40212159661/job/121225534416).
GREEN `1e2d8c08` adds the documentation and [passes
CI](https://github.com/Kpa-clawbot/CoreScope/actions/runs/40212678062),
including all browser shards and architecture builds.
