feat(A.1 #2): build-time CSP regression guard for localhost LLM

Pins the connect-src CSP entries for http://127.0.0.1:* and
ws://127.0.0.1:* at build time. If a future edit to tauri.conf.json
strips the local-LLM permit, the kon crate fails to build rather than
shipping a binary whose webview fetch() silently 404s with an opaque
scope error (Vibe #438 / #487).

Closes item #2 of docs/whisper-ecosystem/brief.md — CSP widening itself
landed in an earlier commit; this is the regression-proofing the plan
calls for.

src-tauri/build.rs

@@ -9,5 +9,35 @@ fn main() {
         println!("cargo:rustc-link-arg=-Wl,--allow-multiple-definition");
     }
 
+    assert_localhost_llm_csp();
+
     tauri_build::build()
 }
+
+/// Regression guard for brief item #2 (pre-emptive localhost LLM scope).
+///
+/// Kon's bundled llama.cpp server and any BYO Ollama install speak HTTP
+/// on `127.0.0.1:*`. If the `connect-src` CSP ever drops those entries,
+/// `fetch()` from the webview to the local LLM silently 404s with an
+/// opaque scope error (Vibe #438 / #487). We keep the current permit
+/// pinned at build time so a stray edit can't regress it unnoticed.
+fn assert_localhost_llm_csp() {
+    let conf_path = std::path::Path::new("tauri.conf.json");
+    println!("cargo:rerun-if-changed=tauri.conf.json");
+
+    let raw = std::fs::read_to_string(conf_path)
+        .expect("build.rs: failed to read tauri.conf.json for CSP regression guard");
+    let csp_start = raw
+        .find("\"csp\"")
+        .expect("build.rs: tauri.conf.json missing \"csp\" field");
+    // Read the rest of the CSP string — we only need to substring-search.
+    let csp_slice = &raw[csp_start..];
+    for required in ["http://127.0.0.1:*", "ws://127.0.0.1:*"] {
+        assert!(
+            csp_slice.contains(required),
+            "build.rs: tauri.conf.json CSP must permit {required} for local LLM \
+             connectivity (brief item #2). Restore the connect-src entry before \
+             building."
+        );
+    }
+}