Say which bytes the byte limits count, and what passing one does (closes #203)

sneak asked for every document to be audited for sentences that do not say what they mean. The byte limits now say which bytes count (response bodies, request bodies or both, as `SWW_BYTES_COUNT` says, WebSocket traffic included) and what passing one does. Other unclear or false sentences in `README.md`, `SPEC.md`, `EVALUATION.md` and `docs/` are rewritten.

Documents only. `REPO_POLICIES.md` is shared policy text and stayed out.

Model: fable-5-1 (first audit); opus-5-5 (rework)

docs/metrics.md

@@ -5,6 +5,9 @@ What `GET /_sww/metrics` answers with, at `discriminator`'s own address.
 keeps the `instance` label as `exported_instance` unless the scrape sets
 `honor_labels: true`. `rproxy` has no metrics.
 
+Every `sww_*request_bytes_total` and `sww_*response_bytes_total` series counts
+body bytes, never headers, whatever `SWW_BYTES_COUNT` says.
+
 ## Traffic
 
 - `sww_requests_total`, `sww_request_bytes_total` and