docs(objectui): replace the phantom field-level visible key with the keys that exist (#13094)

`FieldSchema` and `FormFieldSchema` are strict objects that refuse `visible` by
name, before either documented value shape is reached. Rewrite the two protocol
pages onto the keys that are really declared:

- concept.mdx section 3: `visibleWhen` (CEL predicate over `record`), plus a
  callout naming the polarity trap (`hidden` is INVERTED).
- layout-dsl.mdx "Responsive Layout Modifiers": no field-level breakpoint
  visibility exists in the spec at all, so the `desktop`/`tablet`/`mobile` map
  and the adjacent `portrait`/`landscape` `columns` map are removed rather than
  renamed; the section now documents `responsiveStyles` on a page component,
  which is the only per-breakpoint channel that is actually applied.

No spec change: objectui#6514's maintainer ruling declined adding a field-level
`visible` key to the contract.

Co-authored-by: Claude <noreply@anthropic.com>

Author: Elon Musk

content/docs/protocol/objectui/concept.mdx

@@ -601,7 +601,9 @@ actions:
 
 ### 3. Conditional Field Visibility
 
-Fields appear based on other field values:
+Fields appear based on other field values. The key is **`visibleWhen`**, and its value
+is a **CEL predicate string** evaluated per record — the field is shown only when the
+predicate is TRUE:
 
 ```yaml
 fields:
@@ -611,9 +613,26 @@ fields:
   - name: shipping_address
     type: textarea
     label: Shipping Address
-    visible: { shipping_required: true }  # ← Show only if checkbox checked
+    # Shown only while the checkbox is checked. `has()` guards the unset case:
+    # an unbound reference faults, and visibility's fallback is "shown".
+    visibleWhen: "has(record.shipping_required) && record.shipping_required"
 ```
 
+<Callout type="warn">
+  There is **no `visible` key on a field**. `FieldSchema` is a strict object, so
+  `visible` is refused *by name* — before any value shape is examined — and no
+  filter object, boolean map or breakpoint map makes it parse. Which key you want
+  depends on the form the answer takes, and the two have **opposite polarity**:
+
+  - a **static** answer is `hidden`, which is **inverted** — `visible: false`
+    becomes `hidden: true`;
+  - a **per-record** answer is `visibleWhen`, the CEL predicate above, shown when
+    the predicate is **true**.
+
+  Its siblings are `readonlyWhen` and `requiredWhen`. See the
+  [Field Reference](/docs/references/data/field).
+</Callout>
+
 ### 4. Field Dependencies and Cascading
 
 ```yaml

content/docs/protocol/objectui/layout-dsl.mdx

@@ -521,35 +521,48 @@ tab points at a named list view. See
 
 ## Responsive Layout Modifiers
 
-### Device-Specific Visibility
+**Responsive behaviour is not a form-field or form-section key.** A form field's
+only conditional-visibility key is `visibleWhen` — a CEL predicate over the
+record, so it varies per *record*, never per *viewport* — and a section's
+`columns` is a single scalar (`1`–`4`). Both shapes are strict, so a breakpoint
+or orientation map written on either is a **loud parse error** naming the key,
+not a layout that quietly does nothing.
 
-```yaml
-fields:
-  - name: detailed_description
-    span: 12
-    visible:
-      desktop: true   # Show on desktop
-      tablet: true    # Show on tablet
-      mobile: false   # Hide on mobile
-  
-  - name: short_summary
-    span: 12
-    visible:
-      desktop: false  # Hide on desktop
-      tablet: false   # Hide on tablet
-      mobile: true    # Show on mobile
-```
+### Per-Breakpoint Styling
 
-### Orientation-Specific Layout
+Breakpoint-driven show/hide and per-breakpoint layout live one tier up, on a
+**page component**, and are expressed as scoped CSS through `responsiveStyles`
+([ADR-0065](https://github.com/objectstack-ai/objectstack/blob/main/docs/adr/0065-sdui-styling-model.md)) — desktop-first buckets compiled to id-scoped CSS at
+render:
 
 ```yaml
-section:
-  columns:
-    portrait: 1   # 1 column in portrait mode
-    landscape: 2  # 2 columns in landscape mode
-  fields: [field_a, field_b, field_c]
+# On a page component (*.page.ts) — NOT on a form field or a form section
+responsiveStyles:
+  large:  { gridColumn: 'span 6' }   # unconditional base (desktop-first)
+  medium: { gridColumn: 'span 12' }  # applied at ≤ medium
+  xsmall: { display: 'none' }        # hidden at the smallest breakpoint
 ```
 
+The four buckets are `large` / `medium` / `small` / `xsmall`; values are a
+CSS-property map with camelCase keys. Prefer design tokens
+(`var(--space-6)`) over literals.
+
+<Callout type="warn">
+  Earlier revisions of this page documented a field-level `visible:` breakpoint
+  map (`desktop` / `tablet` / `mobile`) and a section-level `columns:`
+  orientation map (`portrait` / `landscape`). **Neither existed on any schema.**
+  `visible` is not a form-field key at all — `FormFieldSchema` declares `hidden`
+  (static, and **inverted**: `visible: false` is `hidden: true`) and
+  `visibleWhen` (CEL predicate) — and `FormSection.columns` has always been a
+  scalar. Both are **removed rather than implemented**, because field-level
+  breakpoint visibility exists nowhere in the spec: there is no key to rename
+  them onto.
+
+  Nor is `responsive.hiddenOn` the answer. That layout block was retired in
+  v17.x under [ADR-0049](https://github.com/objectstack-ai/objectstack/blob/main/docs/adr/0049-no-unenforced-security-properties.md) D2 precisely because no renderer ever applied it;
+  `responsiveStyles` above is the only per-breakpoint channel that is.
+</Callout>
+
 ## Related Lists: Embedding Child Records
 
 Display child records within a parent record's page.