perf: bound slow ES modules with a DOM-rendered-aware budget

A slow non-essential ES module could block navigation for ~10s. On real
pages (www.ycombinator.com) obscura spent ~11.5s loading YC's bookface
module, whose top-level eval idle-waits forever, until the phase watchdog
killed it. The page renders fully without it (CPU was ~3s, the rest pure
idle), but wall time was ~2.5x lightpanda.

load_module / load_inline_module now take a per-module budget and bound
both the import-graph fetch and the eval loop with it. page.rs sizes the
budget by whether the body is already rendered: an enhancement module on
a populated page (>50 body descendants) gets a short budget (3s, override
OBSCURA_MODULE_BUDGET_MS); an unmounted SPA shell keeps the full script
budget so the app module still mounts (#205). Fetch concurrency is
unchanged; only the idle eval wait is bounded.

www.ycombinator.com: wall 13.1s -> 6.6s, still 475 links, CPU unchanged.
teragrowth.org (Vite SPA) still renders. Obstacle course 33/33 including
the es-modules / ssr-hydrate / spa stages. Post-drop eval works.

crates/obscura-browser/src/page.rs

@@ -572,6 +572,35 @@ impl Page {
             }
         }
 
+        // Per-module budget. Modules on an already-rendered page are
+        // enhancement, not the app: give them a short budget so one slow
+        // non-essential module (e.g. YC's bookface, whose top-level eval
+        // idle-waits ~10s) cannot block navigation completion. A page whose
+        // body is still an empty shell IS the SPA (issue #205), so give it the
+        // full script budget and the app module still mounts.
+        let module_budget_ms: u64 = {
+            let body_nodes = self
+                .js
+                .as_ref()
+                .and_then(|js| {
+                    js.with_dom(|dom| {
+                        dom.query_selector("body")
+                            .ok()
+                            .flatten()
+                            .map(|b| dom.descendants(b).len())
+                            .unwrap_or(0)
+                    })
+                })
+                .unwrap_or(0);
+            let short_ms: u64 = std::env::var("OBSCURA_MODULE_BUDGET_MS")
+                .ok()
+                .and_then(|s| s.parse().ok())
+                .unwrap_or(3_000);
+            // A rendered body has hundreds of descendants; an unmounted Vite/Next
+            // shell is <root> plus maybe a spinner.
+            if body_nodes > 50 { short_ms } else { script_deadline_ms }
+        };
+
         for module_script in &module_scripts {
             if tokio::time::Instant::now() >= script_deadline {
                 tracing::warn!("execute_scripts: deadline reached, skipping remaining module scripts");
@@ -588,7 +617,7 @@ impl Page {
 
                 tracing::info!("Loading ES module: {}", full_url);
                 if let Some(js) = &mut self.js {
-                    match js.load_module(&full_url).await {
+                    match js.load_module(&full_url, module_budget_ms).await {
                         Ok(()) => {
                             tracing::info!("ES module loaded: {}", full_url);
                             self.record_network_event(&full_url, "GET", "Script", 200, &std::collections::HashMap::new(), 0);
@@ -601,7 +630,7 @@ impl Page {
             } else if !module_script.inline.is_empty() {
                 let base = self.url_string();
                 if let Some(js) = &mut self.js {
-                    if let Err(e) = js.load_inline_module(&module_script.inline, &base).await {
+                    if let Err(e) = js.load_inline_module(&module_script.inline, &base, module_budget_ms).await {
                         tracing::warn!("Inline ES module error: {}", e);
                     }
                 }

crates/obscura-js/src/runtime.rs

@@ -563,7 +563,8 @@ impl ObscuraJsRuntime {
         );
         self.object_store.clear();
     }
-    pub async fn load_module(&mut self, url: &str) -> Result<(), String> {
+    pub async fn load_module(&mut self, url: &str, budget_ms: u64) -> Result<(), String> {
+        let budget = tokio::time::Duration::from_millis(budget_ms);
         let specifier = deno_core::ModuleSpecifier::parse(url)
             .map_err(|e| format!("Invalid module URL {}: {}", url, e))?;
 
@@ -585,16 +586,27 @@ impl ObscuraJsRuntime {
             }
         };
 
-        let module_id = self
-            .runtime
-            .load_side_es_module_from_code(&specifier, deno_core::ModuleCodeString::from(source_code))
-            .await
-            .map_err(|e| format!("Module load error: {}", e))?;
+        // Bound the recursive import-graph fetch. deno_core fetches the graph
+        // concurrently, but a module whose top-level eval idle-waits forever (no
+        // CPU, no network) otherwise blocks here until the phase watchdog fires.
+        // The caller sizes the budget: short for enhancement modules on an
+        // already-rendered page, full for an unmounted SPA shell (#205).
+        let module_id = match tokio::time::timeout(
+            budget,
+            self.runtime.load_side_es_module_from_code(&specifier, deno_core::ModuleCodeString::from(source_code)),
+        ).await {
+            Ok(Ok(id)) => id,
+            Ok(Err(e)) => return Err(format!("Module load error: {}", e)),
+            Err(_) => {
+                tracing::warn!("Module graph load timed out after {}ms: {}", budget_ms, url);
+                return Ok(());
+            }
+        };
 
         let result = self.runtime.mod_evaluate(module_id);
 
         let timeout = tokio::time::timeout(
-            tokio::time::Duration::from_secs(10),
+            budget,
             self.runtime.run_event_loop(deno_core::PollEventLoopOptions::default()),
         ).await;
 
@@ -602,7 +614,7 @@ impl ObscuraJsRuntime {
             Ok(Ok(())) => {}
             Ok(Err(e)) => return Err(format!("Module event loop error: {}", e)),
             Err(_) => {
-                tracing::warn!("Module evaluation timed out after 10s: {}", url);
+                tracing::warn!("Module evaluation timed out after {}ms: {}", budget_ms, url);
                 return Ok(());
             }
         }
@@ -616,7 +628,8 @@ impl ObscuraJsRuntime {
         }
     }
 
-    pub async fn load_inline_module(&mut self, code: &str, base_url: &str) -> Result<(), String> {
+    pub async fn load_inline_module(&mut self, code: &str, base_url: &str, budget_ms: u64) -> Result<(), String> {
+        let budget = tokio::time::Duration::from_millis(budget_ms);
         let specifier = deno_core::ModuleSpecifier::parse(
             &format!("{}#inline-module-{}", base_url, self.object_counter),
         )
@@ -624,19 +637,25 @@ impl ObscuraJsRuntime {
 
         self.object_counter += 1;
 
-        let module_id = self
-            .runtime
-            .load_side_es_module_from_code(
+        let module_id = match tokio::time::timeout(
+            budget,
+            self.runtime.load_side_es_module_from_code(
                 &specifier,
                 deno_core::ModuleCodeString::from(code.to_string()),
-            )
-            .await
-            .map_err(|e| format!("Inline module load error: {}", e))?;
+            ),
+        ).await {
+            Ok(Ok(id)) => id,
+            Ok(Err(e)) => return Err(format!("Inline module load error: {}", e)),
+            Err(_) => {
+                tracing::warn!("Inline module graph load timed out after {}ms", budget_ms);
+                return Ok(());
+            }
+        };
 
         let result = self.runtime.mod_evaluate(module_id);
 
         let timeout = tokio::time::timeout(
-            tokio::time::Duration::from_secs(10),
+            budget,
             self.runtime.run_event_loop(deno_core::PollEventLoopOptions::default()),
         ).await;
 
@@ -644,7 +663,7 @@ impl ObscuraJsRuntime {
             Ok(Ok(())) => {}
             Ok(Err(e)) => return Err(format!("Module event loop error: {}", e)),
             Err(_) => {
-                tracing::warn!("Inline module timed out after 10s");
+                tracing::warn!("Inline module timed out after {}ms", budget_ms);
                 return Ok(());
             }
         }