docs(ci): measure the accepted cost instead of comparing it unmeasured

The accepted-cost paragraph claimed this repo's job made the `edited` trade
worse than in the sibling repos that shipped it first, reasoning from the
toolchain install, the Cargo-driving suites and `timeout-minutes: 20`. The
measurement inverts it: last twelve non-skipped runs here are 83 s median
(77-106), against ~118 s for daniel/gitdan and ~330 s for daniel/emowheel --
this is the cheapest of the three, and 20 minutes is a hang ceiling, not a
duration. This PR's own runs measured 87 s and 90 s.

The comparison is dropped rather than re-pointed; the absolute figure replaces
it, with the derivation named (`run_started_at` to `updated_at` off the Actions
API) so a reader can re-take it. The neighbouring concurrency claim was read
off this repo's own file and is unchanged -- it was the checked half of a
paragraph whose other half was not.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LjbhSqQf3pwnPA6MVaWcWL

README.md

@@ -621,14 +621,15 @@ its bare default; either restores the bug.
 **Accepted cost: a body edit on an already-non-draft PR now triggers a real
 run.** Gitea populates no `changes` field for a title-or-body edit, unlike
 GitHub, so the workflow cannot tell the edit that un-drafts a PR from an
-ordinary body PATCH — a closing-reference fix-up, say. That trade is worse
+ordinary body PATCH — a closing-reference fix-up, say. The price is around
-here than in the sibling repos that made it first: this job installs two Rust
+**90 seconds** of a runner shared across four repos on two capacity slots: the
-toolchains and runs suites that drive a real Cargo against real scratch
+last twelve non-skipped runs of this job, `run_started_at` to `updated_at` off
-workspaces, with `timeout-minutes: 20`, on a runner shared across four repos
+the Actions API, are 83 s median over 77–106 s. `timeout-minutes: 20` is a
-on two capacity slots. It is bounded, though: the workflow's `concurrency:`
+ceiling for a hung suite, not a duration. It is also bounded: the workflow's
-block groups `pull_request` runs on `github.ref` with `cancel-in-progress:
+`concurrency:` block groups `pull_request` runs on `github.ref` with
-true`, so a burst of edits collapses to one run rather than N. A still-draft
+`cancel-in-progress: true`, so a burst of edits collapses to one run rather
-PR pays nothing extra — the `if:` guard skips those exactly as before.
+than N. A still-draft PR pays nothing extra — the `if:` guard skips those
+exactly as before.
 
 The evidence for `edited` comes from `daniel/emowheel`, which hit the identical
 bug, shipped the identical wrong fix, and corrected it in commit `08da820` (see