Unify scope resolution: walk_scope, resolve_binder, free_vars into single compiler phase

[dowdiness]

Problem

Three functions solve overlapping scope resolution problems with different approaches:

Function	Location	Approach	Handles nested blocks?
walk_scope	examples/ideal/main/scope_annotation.mbt	Recursive walk with scope stack	yes
resolve_binder	lang/lambda/edits/scope.mbt	Ad-hoc FlatProj + source positions	no
free_vars	lang/lambda/edits/free_vars.mbt	Recursive walk with bound-name set	yes

walk_scope and free_vars follow the same pattern (recursive descent maintaining scope context) and handle nested blocks correctly. resolve_binder is ad-hoc and broken for nested blocks.

Ideal architecture

A single scope resolution phase in lang/lambda/ that produces a binding map:

Input:  ProjNode tree (or Term)
Output: Map[NodeId, BindingInfo]  — every Var → its binder

This replaces:

• resolve_binder — just look up the binding map
• free_vars — vars not in the binding map are free
• walk_scope in the ideal editor — reads the binding map and adds UI metadata (color, usage_ids)

Dependencies

• Architecture: add LetDef ProjNode to give module bindings first-class identity #127 (LetDef ProjNode) would give module defs proper NodeIds, eliminating synthetic IDs in the binding map
• Without Architecture: add LetDef ProjNode to give module bindings first-class identity #127, the phase still works but needs synthetic IDs for module-def binders (current walk_scope approach)

Scope

• Move walk_scope's algorithm to lang/lambda/edits/ as a proper compiler phase
• Replace resolve_binder call sites (scope.mbt, text_edit_refactor.mbt)
• Replace free_vars with a derived query on the binding map
• Keep walk_scope in the ideal editor as a thin wrapper that reads the phase output