docs(smoke): point each smoke's on-demand example at its own command

smoke-web-elicitation.mjs told the reader to run
`SMOKE_BROWSER=webkit npm run smoke:web:app` -- the App smoke, not the
elicitation smoke the file documents. Following it exercises a different
path entirely, and it does not fail while doing so: a real smoke runs and
passes, so nothing signals that you never tested what you were reading
about.

This is a direct cost of my own decision two rounds ago to make the two
App headers word-for-word identical so they could not drift. The shared
PROSE should be identical; the example command is the one line that must
not be, and copy-paste does not distinguish them. Both headers now name
their own smoke, and both point at npm run smoke:web:webkit for all three.

Guarded, since the class is invisible at runtime by construction:
run-engine-smokes.test.mjs now asserts no smoke header tells the reader to
run a sibling's command, and that every `npm run` it does name is a real
script. Both mutation-verified -- restoring the exact bug fails the first,
and naming a nonexistent script fails the second.

Addresses the suppressed comment on Copilot review 5031065042 of #2133.

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

scripts/smoke-web-app.mjs

@@ -50,7 +50,8 @@
  *   - **`npm run ci`**, the local pre-push gate, runs it in **Chromium and
  *     Firefox** — the Firefox pass is `smoke:web:firefox`, and it is the one
  *     gate step with no GitHub CI counterpart.
- *   - **WebKit is on demand only**: `SMOKE_BROWSER=webkit npm run smoke:web:app`.
+ *   - **WebKit is on demand only**: `SMOKE_BROWSER=webkit npm run smoke:web:app`
+ *     for this smoke alone, or `npm run smoke:web:webkit` for all three.
  *
  * Firefox passes. WebKit fails this smoke for reasons nobody has identified and
  * nobody is investigating: it does not reproduce in real Safari, and an isolated