feat(16.2-01): translatable plugin field types, untouched skip, writes, list values and uploads

- translatable types lift, store and hydrate through the ML path
  (isMLField); Decode and Load run per locale and notices carry the locale
- on update a value equal to the served value (Load of the stored host
  value or exact translation) is neither decoded nor written
- FieldWrite, FieldWritesFromContext and WithFieldWrites expose what a save
  writes to the Form hooks
- list columns may use a plugin alias and show the type's ListValue
- POST .../{id}/fields/{field}/upload hands the file to the type's Upload
  (guard, X-Requested-With, permissions, field context, record scope, body
  cap); OpenAPI, route inventories, README and docs updated

modules/cabana/tx_context.go

@@ -65,3 +65,56 @@ func VirtualFieldsFromContext(ctx context.Context) (map[string]any, bool) {
 	}
 	return out, true
 }
+
+type fieldWritesKey struct{}
+
+// FieldWrite is one value a save wrote for a plugin field type
+// (pact.HasFormFieldTypes): Locale is the content locale of a translatable
+// type's value and empty otherwise, Default is true when the value went to
+// the host column, and Value is what the type's Decode returned.
+type FieldWrite struct {
+	Locale  string
+	Default bool
+	Value   any
+}
+
+// withFieldWrites returns ctx carrying the plugin field values a save writes.
+func withFieldWrites(ctx context.Context, writes map[string][]FieldWrite) context.Context {
+	if writes == nil {
+		writes = map[string][]FieldWrite{}
+	}
+	return context.WithValue(ctx, fieldWritesKey{}, writes)
+}
+
+// WithFieldWrites returns ctx carrying writes as a save would, so a test can
+// call a Form hook that reads FieldWritesFromContext. The map is copied.
+func WithFieldWrites(ctx context.Context, writes map[string][]FieldWrite) context.Context {
+	return withFieldWrites(ctx, copyFieldWrites(writes))
+}
+
+// FieldWritesFromContext returns the values a create or update save
+// actually writes for the form's plugin field types, keyed by field name, for
+// the context handed to a Form hook (pact.FormBeforeCreate,
+// pact.FormAfterCreate, pact.FormBeforeUpdate, pact.FormAfterUpdate). A field
+// or locale whose submitted value equals the value the form was served is
+// not written and not listed; a translatable type lists one entry per
+// written locale. The map is a copy. The second result is false outside a
+// create or update save.
+func FieldWritesFromContext(ctx context.Context) (map[string][]FieldWrite, bool) {
+	if ctx == nil {
+		return nil, false
+	}
+	writes, ok := ctx.Value(fieldWritesKey{}).(map[string][]FieldWrite)
+	if !ok {
+		return nil, false
+	}
+	return copyFieldWrites(writes), true
+}
+
+func copyFieldWrites(in map[string][]FieldWrite) map[string][]FieldWrite {
+	out := make(map[string][]FieldWrite, len(in))
+	for field, list := range in {
+		out[field] = append([]FieldWrite(nil), list...)
+	}
+	return out
+}