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/lsp/server.zig

@@ -1749,6 +1749,7 @@ pub const Server = struct {
                 .err => 1,
                 .warn => 2,
                 .note => 3,
+                .help => 4,
             };
             result.append(allocator, .{
                 .range = range,