docs: explain reusable partials

Add the public 'Where are my components?' answer: hemx maps framework component jobs to hemplate partials, generated helpers, app-owned state, IntoEffect composition, and explicit leaf islands. The pre-commit guard now only blocks commits that change REQUIREMENTS.md without an AGENTS.md review, so unrelated follow-up commits are not trapped by old history.

req: canonical_authoring/002

req: canonical_authoring/003

docs/recipes/reusable-partials.md

@@ -0,0 +1,41 @@
+# Where are my components?
+
+In hemx, the reusable UI unit is a **checked hemplate partial plus generated Rust
+helpers**, not a client component instance. You still get reuse and composition;
+the ownership moves to places Rust apps can inspect and test. req: canonical_authoring/002 req: canonical_authoring/003
+
+| Framework component job | hemx home |
+| --- | --- |
+| Markup and local UI shape | A `.heml` partial rendered from a Rust view struct. |
+| Props | The view struct fields passed into the partial/helper. |
+| Stable child identity | `h-key` on repeated partials, exposed through generated keyed helpers. |
+| Events | Real forms, links, handles, and explicit generated events. |
+| State | App-owned Rust state, commands/events/projections, or integration-owned stores. |
+| Updating the UI | Generated commands such as `ui::todo_row.replace(row)`. |
+| Composition | `impl IntoEffect`: tuples for fixed mixed batches, arrays for fixed repeated batches, and `Vec<T: IntoEffect>` for dynamic repeated batches. |
+| Client-only widgets | Explicit islands or Web Components at leaf boundaries. |
+
+A reusable row should be one partial used in both places: initial render and later
+updates. The handler builds domain state, converts it to a view value, and returns
+generated commands:
+
+```rust
+(
+    rows
+        .into_iter()
+        .map(|row| ui::todo_row.replace(row))
+        .collect::<Vec<_>>(),
+    ui::summary.set(summary),
+    ui::notice.set("Saved"),
+)
+```
+
+That is the component story: the row partial is reusable; the generated helper
+knows the target and swap kind; `IntoEffect` composes the update without a client
+component runtime, selector lookup, raw ids, raw opcodes, or manual registry
+plumbing. req: public_api/005
+
+Use an island only when the browser must own high-frequency local behavior, such
+as a chart, map, editor, or media widget. The island is an explicit leaf; it can
+emit facts back through generated handles/events, but the app still changes
+server-owned UI through normal hemx effects. req: interop/003