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.

tests/expected/56-err-tuple-oob.txt

@@ -1 +1,5 @@
-/Users/agra/projects/sx/examples/56-err-tuple-oob.sx:6:15: error: field '42' not found on type 'tuple'
+error: field '42' not found on type 'tuple'
+  --> /Users/agra/projects/sx/examples/56-err-tuple-oob.sx:6:15
+   |
+ 6 |     return xx t.42;
+   |               ^^^^