Make a type-annotated binder shadow a like-named constant

[kings-crown]

A bound variable may legitimately share its name with a constant, and
the binder should win within its scope. This already worked for
un-annotated varstructs via ignore_constant_varstruct, but broke as soon
as the binder carried a type annotation:

new_definition `test_const = T`;;            (* test_const : bool *)
`forall (test_const:num). test_const > 0`;;
(* typechecking error (initial type assignment): test_const has
   type bool, it cannot be used with type num *)

Cause: in typify's Absp branch the varstruct was typed first, and only
then was a resulting Constp rewritten back to a Varp. With an annotation
the constant-resolution clause unifies the constant's generic type
(bool) against the annotation (num) and raises before that post-hoc
rescue can run.

Fix: seed the varstruct's constant-named binders into venv before typing
it, so they take typify's "already a bound variable" clause rather than
being resolved to the like-named constant. This generalizes the old
rescue: it also covers type-annotated binders such as
\(Q1:num#num#num#num). ... and tuple varstructs. Only binder-position
constant names are seeded, so free occurrences still resolve to the
constant. With ignore_constant_varstruct=false the previous behaviour is
preserved exactly.

Patch constructed with the help of (mostly) Claude Code.