lang F0.3: multi-message diagnostic bundling + help-blocks

Feature 0 complete. addNote/addHelp bundle notes and help-blocks under a
primary diagnostic (handle from new addId/addFmtId); help blocks carry an
optional fix-it line that substitutes the suggested source. renderExtended
now renders primary -> notes -> helps with blank-line separators.

Wire the CLI to the extended renderer (renderErrors -> renderStderr) and
flip render_style default to .extended; the previous renderErrors ->
renderDebug path bypassed render() entirely, so flipping the field alone
was a no-op. 13 diagnostic snapshots re-rendered to the extended format.

src/core.zig

@@ -371,6 +371,6 @@ pub const Compilation = struct {
     }
 
     pub fn renderErrors(self: *const Compilation) void {
-        self.diagnostics.renderDebug();
+        self.diagnostics.renderStderr();
     }
 };