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.
`compile_error(msg)` raises a build-time diagnostic at the call site
with `msg` as the error text. The arg must be a string literal —
runtime expressions can't be reported as compile errors. Used by
builder fns to reject malformed pack shapes / arg combinations
cleanly instead of silently emitting wrong code.
Today: `unresolved 'compile_error'`. Expected (post-fix): focused
diagnostic with the literal message at the call site's span. The
next commit adds the lowering arm.
