v2.1 task 01: state the ReverseProxy facts (ErrorHandler before headers, ErrAbortHandler mid-body); note the refusal-ending

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/plans/v2.1/01-cancel-record.md

@@ -23,6 +23,23 @@ responses under the given test's load; `TestQueueFullIs503` flaked on it. The re
 `ErrorHandler` already sees `context.Canceled` for the "client gone before the response" case
 and currently returns without leaving a trace, which is why the post-hoc check was there.
 
+## Facts about `httputil.ReverseProxy` (Go 1.26) — you cannot read its source from here
+
+The standard library lives outside the repository and the sandbox refuses reads there; do not
+try. What you need:
+
+- `ServeHTTP` calls `ErrorHandler(w, req, err)` when the outgoing request fails **before any
+  response byte was written** — for a client that left, `err` satisfies
+  `errors.Is(err, context.Canceled)`. After the response headers were written, `ErrorHandler`
+  is never called.
+- If copying the response body to the client fails (the client left mid-body), `ServeHTTP`
+  **panics with `http.ErrAbortHandler`**; v1.1's deferred `recover` in `forward.go` already
+  turns that into the 499 row and re-panics.
+- `ServeHTTP` returning normally therefore means the response was delivered in full (or
+  `ErrorHandler` answered). The request's context may nonetheless already be cancelled at that
+  moment — the server cancels it when the client's connection closes — which is exactly the
+  signal the current code misreads.
+
 ## Files
 
 - Copy: `internal/proxy/served_test.go`