feat(host): add typed host capability boundary

Introduce hemx-host with typed capability manifests, calls, events, host-check failures, reusable browser/PWA and native-shell profiles, and a tiny optional browser adapter for haptics/share that returns host events without DOM mutation.

req: host/001

req: host/002

req: host/003

req: host/004

req: host/005

REQUIREMENTS.md

@@ -484,6 +484,25 @@ what a valid business email is.
 
 ---
 
+## host
+
+### req: host/001
+001	Host capabilities are declared as typed capability uses with one of four shapes: fire, request, stream, or schedule. This contract covers browser, PWA, WebView, and native-shell hosts without adding a second UI runtime.
+
+### req: host/002
+002	Host adapters may produce host events or perform explicit host side effects, but they must not mutate DOM, own application/domain state, append domain events, bypass generated hemx effects, or introduce a client app-state framework.
+
+### req: host/003
+003	Permission-sensitive host capabilities require an explicit user-facing reason in the app-owned manifest before standard host checks may pass.
+
+### req: host/004
+004	Host checks report concrete failures for undeclared capability use, unsupported host capability shape, and missing permission reasons before a host adapter executes the capability call.
+
+### req: host/005
+005	Host results return to app code as facts. App/domain code decides whether they become commands, events, persistence, or UI effects; hemx UI updates still happen through normal EffectBatch output.
+
+---
+
 ## auth
 
 ### req: auth/001