Feat v0.9.5 typed forms SDK primitive

Extract typed form system from models/workflow.go into standalone
server/forms/ package. Any package can now declare and validate
forms, not just workflow stages.

- New server/forms/ package (types + validation + condition evaluator)
- REST POST /api/v1/forms/validate endpoint
- Starlark forms.validate(template, data) module
- Frontend sw.forms.render(), .validate(), .validateRemote()
- Manifest form_template accepted at package level
- 16 new tests (12 forms + 4 Starlark module)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/main.go

@@ -545,6 +545,10 @@ func main() {
 			protected.POST("/instances/:iid/signoffs", wfSignoffH.Submit)
 			protected.GET("/instances/:iid/signoffs", wfSignoffH.List)
 
+			// Forms validation
+			formsH := &handlers.FormsHandler{}
+			protected.POST("/forms/validate", formsH.Validate)
+
 			// Surface discovery
 			pkgH := handlers.NewPackageHandler(stores)
 			protected.GET("/surfaces", pkgH.ListEnabledSurfaces)