Dev loop: debug-only page scripts from disk + tab rebuild on change (waybetter-host devloop, page_script!), generic devmode watcher, waybetter dev, buildRuntime watch, template scripts

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/waybetter-host/src/lib.rs

@@ -24,6 +24,8 @@
 //!   shows, its page color and delivery script) over a site's built-in themes.
 //! - [`workspaces`]: saved tab/pane/deck layouts, bounded and well-formed (never evaluated).
 //! - [`devmode`]: developer mode's debounced watcher of plugins, custom CSS and a theme file.
+//! - `devloop` (debug builds with `app` only): page scripts read from disk and every tab rebuilt when
+//!   one changes; an app names its files with [`page_script!`].
 //! - [`settings`]: the shell's settings (home page, where links open, session restore) and the page's
 //!   feature flags, as validated data and the scripts that hand them to site tabs.
 //! - [`modes`]: the onboarding mode the owner started from (the presets are the app's feature table).
@@ -74,7 +76,55 @@ pub mod zoom;
 
 #[cfg(feature = "app")]
 pub mod app;
+#[cfg(all(feature = "app", debug_assertions))]
+pub mod devloop;
 #[cfg(feature = "tauri")]
 pub mod host;
 #[cfg(all(feature = "tauri", feature = "net"))]
 pub mod updater;
+
+/// An app's page script for `SiteHooks::init_scripts`: `$embedded` (an `include_str!`) in every
+/// build, except that a debug build reads `$file` (relative to the app crate's manifest directory)
+/// from disk each time and falls back to the embedded copy when it is missing. The files named
+/// this way are watched: a change rebuilds every tab (`devloop`). In a release build the disk arm
+/// is not compiled at all: the expansion is `$embedded.to_owned()`, `$file` appears nowhere.
+#[macro_export]
+macro_rules! page_script {
+    ($embedded:expr, $file:literal) => {{
+        #[cfg(debug_assertions)]
+        {
+            $crate::devloop::from_disk(concat!(env!("CARGO_MANIFEST_DIR"), "/", $file), $embedded)
+        }
+        #[cfg(not(debug_assertions))]
+        {
+            ($embedded).to_owned()
+        }
+    }};
+}
+
+#[cfg(test)]
+mod release_guard {
+    /// Release builds must not contain the dev loop: every mention of it in this crate is behind
+    /// `cfg(debug_assertions)` (checked on the source, so it also holds when only debug is built).
+    #[test]
+    fn every_devloop_mention_is_debug_only() {
+        for (file, source) in [("lib.rs", include_str!("lib.rs")), ("app.rs", include_str!("app.rs"))] {
+            let lines: Vec<&str> = source.lines().collect();
+            for (index, line) in lines.iter().enumerate() {
+                let code = line.trim_start();
+                if code.starts_with("//") || !(code.contains("devloop::") || code.starts_with("pub mod devloop")) || code.contains("$crate::devloop") {
+                    continue;
+                }
+                let gated = (index.saturating_sub(2)..index).any(|before| lines[before].contains("debug_assertions"));
+                assert!(gated, "{file}:{}: devloop used outside cfg(debug_assertions)", index + 1);
+            }
+        }
+    }
+
+    /// `cargo test --release`: the macro is just the embedded text.
+    #[cfg(not(debug_assertions))]
+    #[test]
+    fn release_page_script_is_the_embedded_copy() {
+        assert_eq!(crate::page_script!("embedded", "no/such/file.js"), "embedded");
+    }
+}