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/errors.test.zig

@@ -237,12 +237,116 @@ test "renderExtended: multi-line span renders each line with carets" {
     try testing.expectEqualStrings(expected, output);
 }
 
-test "render: default compact style still works" {
+test "renderExtended: primary bundled with one note" {
+    var dl = errors.DiagnosticList.init(testing.allocator, "let x = y\nlet y = z\n", "b.sx");
+    defer dl.deinit();
+    const p = dl.addId(.err, "undefined name", Span{ .start = 8, .end = 9 });
+    dl.addNote(p, Span{ .start = 14, .end = 15 }, "declared here");
+
+    const output = try renderToString(&dl, testing.allocator);
+    defer testing.allocator.free(output);
+
+    const expected =
+        \\error: undefined name
+        \\  --> b.sx:1:9
+        \\   |
+        \\ 1 | let x = y
+        \\   |         ^
+        \\
+        \\note: declared here
+        \\  --> b.sx:2:5
+        \\   |
+        \\ 2 | let y = z
+        \\   |     ^
+        \\
+    ;
+    try testing.expectEqualStrings(expected, output);
+}
+
+test "renderExtended: primary bundled with help (no fix code)" {
+    var dl = errors.DiagnosticList.init(testing.allocator, "foo bar\n", "h.sx");
+    defer dl.deinit();
+    const p = dl.addId(.err, "bad thing", Span{ .start = 0, .end = 3 });
+    dl.addHelp(p, null, "try something else", null);
+
+    const output = try renderToString(&dl, testing.allocator);
+    defer testing.allocator.free(output);
+
+    const expected =
+        \\error: bad thing
+        \\  --> h.sx:1:1
+        \\   |
+        \\ 1 | foo bar
+        \\   | ^^^
+        \\
+        \\help: try something else
+        \\
+    ;
+    try testing.expectEqualStrings(expected, output);
+}
+
+test "renderExtended: help with fix-it code substitutes the line and omits arrow" {
+    var dl = errors.DiagnosticList.init(testing.allocator, "  x := foo.value\n", "f.sx");
+    defer dl.deinit();
+    // primary span covers "value" at columns 12..17.
+    const p = dl.addId(.err, "no such field", Span{ .start = 11, .end = 16 });
+    dl.addHelp(p, Span{ .start = 11, .end = 16 }, "did you mean `val`?", "  x := foo.val");
+
+    const output = try renderToString(&dl, testing.allocator);
+    defer testing.allocator.free(output);
+
+    const expected =
+        \\error: no such field
+        \\  --> f.sx:1:12
+        \\   |
+        \\ 1 |   x := foo.value
+        \\   |            ^^^^^
+        \\
+        \\help: did you mean `val`?
+        \\   |
+        \\ 1 |   x := foo.val
+        \\   |            ^^^^^
+        \\
+    ;
+    try testing.expectEqualStrings(expected, output);
+}
+
+test "renderExtended: note and help bundle in note-then-help order" {
+    var dl = errors.DiagnosticList.init(testing.allocator, "aaa\n", "o.sx");
+    defer dl.deinit();
+    const p = dl.addId(.err, "primary", Span{ .start = 0, .end = 3 });
+    // Add help first, then note: rendering must still emit note before help.
+    dl.addHelp(p, null, "the help", null);
+    dl.addNote(p, Span{ .start = 0, .end = 3 }, "the note");
+
+    const output = try renderToString(&dl, testing.allocator);
+    defer testing.allocator.free(output);
+
+    const expected =
+        \\error: primary
+        \\  --> o.sx:1:1
+        \\   |
+        \\ 1 | aaa
+        \\   | ^^^
+        \\
+        \\note: the note
+        \\  --> o.sx:1:1
+        \\   |
+        \\ 1 | aaa
+        \\   | ^^^
+        \\
+        \\help: the help
+        \\
+    ;
+    try testing.expectEqualStrings(expected, output);
+}
+
+test "render: compact style still available behind the flag" {
     var dl = errors.DiagnosticList.init(testing.allocator, "abc\n", "c.sx");
     defer dl.deinit();
+    dl.render_style = .compact;
     dl.add(.err, "compact error", Span{ .start = 0, .end = 3 });
 
-    // Default render_style is .compact — preserves the existing gcc-style.
     var aw = std.Io.Writer.Allocating.init(testing.allocator);
     defer {
         var r = aw.writer.toArrayList();