docs(smoke): finish the CI-topology correction in README and the workflow

Round 1 fixed the three smoke headers and AGENTS.md but left the same
overstatement in the two files Copilot flagged as previously-missed:

- README said the matrix runs 'the remaining engines', which reads as
  Firefox AND WebKit and contradicts the very next paragraph. It now
  names the current matrix (Firefox alone) and points at that paragraph.
- The workflow's fail-fast comment cited a WebKit-vs-Firefox scenario the
  one-entry matrix cannot produce. It now justifies keeping the flag for
  when a second engine returns, matching the AGENTS.md wording.

Swept the whole tree for the remaining variants of the claim; none left.

Addresses the suppressed comments on Copilot's second review of #2133.

Signed-off-by: cliffhall <cliff@futurescale.com>

.github/workflows/main.yml

@@ -141,9 +141,10 @@ jobs:
   browser-engine-smokes:
     runs-on: ubuntu-latest
     strategy:
-      # Report every engine's verdict. Failing fast would hide a WebKit-only
-      # regression behind a Firefox-only one, which is the exact distinction
-      # this job exists to draw.
+      # Kept on despite the matrix currently holding one engine: the moment a
+      # second is added back (see below), failing fast would hide one engine's
+      # regression behind another's — which is the exact distinction this job
+      # exists to draw, so the flag should not have to be remembered then.
       fail-fast: false
       matrix:
         # `webkit` belongs here and is deliberately absent: the smokes RUN in it