docs(adr-068): GW-15 body-limit layer + pre-extend cap in publish body handling

docs/architecture/decisions/068-gateway-publish-endpoint.md

@@ -72,8 +72,16 @@ stream; the operation's final `ResponseEnvelope` is the HTTP response.
 The NDJSON body is **streamed, never fully buffered** (GW-06): axum's
 `Body` is framed into lines as bytes arrive, and each line is parsed and
 pushed into the sink lazily — memory is bounded by the per-line cap (2
-MiB, matching axum's default whole-body limit), not by the unbounded
-chunk count. `publish_schema` validation is applied per chunk inside the
+MiB), not by the unbounded chunk count. The per-line cap is checked
+**before** the unterminated byte buffer is extended (GW-15), so it holds
+even when no `\n` ever arrives, and the trailing-EOF path re-checks it
+before yielding. The gateway router also carries an explicit whole-body
+limit layer (GW-15: 2 MiB + 64 KiB framing headroom, answering `413`) —
+a raw-`Body` handler never consults axum's `DefaultBodyLimit` (that
+limit is an extension extractors read), so the layer is the real
+backstop; the layer deliberately sits above the per-line cap so a single
+over-cap line still surfaces the route's semantic line-cap error rather
+than the plain-text 413. `publish_schema` validation is applied per chunk inside the
 sink-feeding stream (GW-01), so a Pub op registered with a
 `publish_schema` enforces the same per-chunk contract over HTTP as over
 the call protocol; a violation terminates the chunk stream with
@@ -139,11 +147,13 @@ the doc does not preload operations.
   the `/openapi.json` version bumps~~ — settled: first-line
   `{operation, chunk}` convention; terminal errors are plain HTTP
   status + JSON body (not an NDJSON line).
-- The 2 MiB per-line cap (not a whole-body cap) bounds a single chunk;
-  the total number of chunks is unbounded. Handlers that would receive
-  unbounded streams over the wire get the same behavior over HTTP —
-  operators front the endpoint with the same body/timeout controls used
-  for any other streaming surface.
+- The 2 MiB per-line cap bounds a single chunk (and, pre-newline, the
+  unterminated buffer itself — GW-15); the total number of chunks is
+  unbounded but the whole request body is capped by the gateway
+  body-limit layer (2 MiB + 64 KiB headroom → `413`, GW-15). Handlers
+  that would receive unbounded streams over the wire get the same
+  behavior over HTTP — operators front the endpoint with the same
+  timeout controls used for any other streaming surface.
 
 ## References