docs(requirements): split island boundary rule

Split oversized canonical_authoring/007 into explicit leaf adapter and no-second-UI-model requirements.

req: canonical_authoring/007

req: canonical_authoring/017

AGENTS.md

@@ -65,6 +65,7 @@ Keep it stable. Prefer pointers to canonical sources over copied structure, file
 - The stable public `.heml` authoring surface lives in `docs/hemplate-syntax.md`; Hemlate examples must use that real hemplate syntax, not Vue/Handlebars sketches.
 - Optional `.heml` editor overlays must share authority with `hemx-build` diagnostics and `docs/hemplate-syntax.md`; `hemx-lsp` owns editor protocol glue for diagnostics/completion/hover and derive-known template facts, while VS Code/Cursor/Neovim keep normal HTML/tree-sitter tooling. Do not create a second template language, selector model, formatter, Rust type system, or custom editor framework. req: diagnostics/004 req: diagnostics/005 req: diagnostics/006
 - JS runtime changes must preserve root-scoped lookup, fail-closed request handling, root-scoped error outlets, and tiny pending/failure/trigger-timing conventions without selectors, VDOM, expressions, or per-node listeners. req: runtime/005 req: convention/001 req: convention/003 req: convention/005 req: convention/007
+- Opaque island JavaScript is a leaf adapter for high-frequency local behavior only; it must not introduce a component runtime, client state graph, VDOM, or second UI model. req: canonical_authoring/017
 - Host capability adapters must stay at the `hemx-host` boundary: they may call host APIs and return host events, but they must not mutate DOM or own app/domain state. req: host/002
 - Local/offline app behavior should be commands/events/projections; do not add `hemx-local`, stored DOM patches, or stored `EffectBatch` truth without a proven reusable contract. req: local/001 req: local/002
 - Axum apps should serve and load the shared runtime through hemx-axum helpers such as `runtime_js_path()` and `runtime_js()`, not hard-coded `/hemx.js` URLs or app-owned cache-busting strings.

README.md

@@ -89,7 +89,7 @@ req: failure/004 req: derive_handler/004
   checked, but hemx does not introduce a styling runtime.
 - Custom JavaScript belongs at explicit opaque leaf boundaries: charts, maps,
   editors, Web Components, or similar widgets. Islands communicate through
-  generated handles/events and do not create a second UI model. req: canonical_authoring/007 req: interop/003
+  generated handles/events and do not create a second UI model. req: canonical_authoring/007 req: canonical_authoring/017 req: interop/003
 
 ## Production boundary
 

REQUIREMENTS.md

@@ -92,7 +92,10 @@ client app state framework.
 013	Explicit primitives, raw targets, raw HTML, raw effects, manual registries/form parsing, low-level ids/opcodes, wire formats, and raw routes remain named escape hatches or internals around the same render/target/effect/transport model. [north_star]
 
 ### req: canonical/007
-007	Opaque islands are explicit leaf adapters: templates declare `data-hemx-island` and optional generated handles/events; server code may emit snapshots/events such as `ui::game.emit(event)`, while island JS owns only high-frequency local behavior. Islands do not introduce a component runtime, client state graph, VDOM, or second UI model. [north_star]
+007	Opaque islands are explicit leaf adapters: templates declare `data-hemx-island` and optional generated handles/events; server code may emit snapshots/events such as `ui::game.emit(event)`. [north_star]
+
+### req: canonical/017
+017	Island JavaScript owns only high-frequency local behavior and must not introduce a component runtime, client state graph, VDOM, or second UI model. [north_star]
 
 ### req: canonical/008
 008	Offline/PWA support is opt-in adapter territory. Server-first hemx may fail interactions while offline; cached shells and local-sync queues live in crates such as `hemx-pwa` or `hemx-sync`, reuse generated slots/effects, queue explicit patches, and reconcile with server-canonical effects. Core hemx must not gain a mandatory client state graph, scheduler, CRDT, or local app runtime. [north_star]