ui takes the root namespace and replaces print and fail #698

Epic: #445 — The Starlark authoring surface
Plan: docs/plans/ui-at-root.md

Problem

Starlark ships two output builtins — print and fail — and devlore owns neither.

A script's print resolves to starlark.Universe, is formatted by the interpreter, and is written straight
to stderr by starlark-go's default. It never reaches status.Narrator, so it is invisible to --quiet, to
any output shaping we choose, and to the diagnostics stream that #507 will introduce. fail is the same
story on the error path.

Meanwhile ui already has methods that mean exactly these things, and five more besides.

The shape of the fix

Marking the ui provider +devlore:root=true surfaces all six methods as top-level globals. Because
Starlark's resolver checks predeclared before universal (resolve.go:437 precedes :441), and
CamelToSnake maps Print→print and Fail→fail, the builtins are replaced — with no interception
code, no starlark.Universe mutation, and no Thread.Print hook.

receiver_type.go:66 already documents this case, using note() as its example. This would be the first
real use of the RoleModule|RoleRoot branch that runtime.go:125 calls reserved.

What it buys

Prerequisite

ui.Print(msg string) takes one string. The builtin is variadic with a sep keyword. Replacing the
builtin with a narrower signature would break every print(a, b) in the tree — so the variadic change
lands first (see the task below).

Ruling needed

Does error belong in the global namespace? note, warn, succeed, print, and fail are safe
words to claim. error is an ordinary word for a script to want as a variable, and a predeclared name
cannot be assigned over at module scope — error = compute() becomes a resolve error, not a shadow.
Options: accept it, give the global a different name while keeping ui.Error, or leave Error off root.

Out of scope, but noted

• The other builtins — dir, getattr, hasattr, type, repr — stay the universe's. Three are
  reflection over the surface, which is a real question for a hermetic runtime, and a separate one.
• Eight sites construct starlark.Thread independently and none sets Thread.Print. Irrelevant once
  print is predeclared (the builtin is never reached), but any policy needing the thread hook would
  have to set it eight times.