docs: record that Set Fields throws instead of dropping data

docs/superpowers/specs/2026-08-04-tier-1-nodes-design.md

@@ -120,7 +120,7 @@ re-interpolate.
 
 | File | Node | Ports | Config |
 | --- | --- | --- | --- |
-| `set-fields.js` | Set / Edit Fields | main | `fields: [{name, value}]`, `mode: keep-all \| only-set`. Dot paths in `name` build nested objects |
+| `set-fields.js` | Set / Edit Fields | main | `fields: [{name, value}]`, `mode: keep-all \| only-set`. Dot paths in `name` build nested objects. Throws rather than quietly losing data: keep-all requires an object input, and a dotted path refuses to overwrite a non-object it passes through |
 | `switch.js` | Switch | dynamic `case0..n` + `fallback` | `field`, `rules: [{label, operator, value}]`. First match wins, sets `_activeBranch` |
 | `filter.js` | Filter | `kept`, `discarded`, both live | Condition list and operators matching `if-condition`, applied per array item |
 | `merge.js` | Merge | main, two named inputs | `mode: append \| combine-by-key \| choose-first`, `key` for combine |