Resolves an :el-id value into the id string rendered into the DOM.

Callers never build strings themselves. Components and the SDK both call
`el-id`, so a view and a test using the same value always agree:

  :first-name                     -> "first-name"
  :app.person/first-name          -> "app.person/first-name"
  [:person "Bla" "ble"]           -> "person--bla--ble"
  [some-element-fn "Bla"]         -> whatever (some-element-fn "Bla") resolves to
  "already-built"                 -> "already-built"

Suffixes are joined with "--" so scanned ids can be split back into a
nested path.

El-id data for gallery demos. Returns plain :el-id values (vectors), so a
view passes them straight to a primitive's :el-id and a test passes the
same value to the SDK; neither builds an id string.

  (el :autocomplete :basic)          -> [:autocomplete :basic]           ; "autocomplete--basic"
  (el :slider :basic :readout)       -> [:slider :basic :readout]        ; "slider--basic--readout"
  (testid :slider :basic :readout)   -> "slider--basic--readout"         ; for raw :data-testid

Read every element carrying an el-id and nest the readings by el-id path.

Shared by the SDK (runs the scripts in the browser via Playwright) and the
gallery (runs them directly), so both see exactly the same snapshot.

  "first-name"         -> {:first-name {:type :input :value "John" :label "First name"}}
  "person--bla--ble"   -> {:person {:bla {:ble {:type :text :text "Ble"}}}}

Each leaf says what it is (:type). Controls carry their :value (and :label
when the control has one); everything else carries its visible :text.