typecheck: a _ctx-first container that returns its handle is not a block setter

[nicolas-maman]

Why

Moving aether-ui from 0.645 to 0.681 (aether-lang-dev/aether-ui#147) fails every example in the fan-out with:

error[E0200]: 'vstack' is a block setter, not a node builder; call it inside a builder's block (e.g. `mod.bundle() { vstack() }`), not as a top-level `ui.vstack() { ... }`
  --> examples/counter/counter.ae:14:11

E0200 (b551fe5, 0.667) discriminates a block setter from a DSL container by "the callee's module also defines a builder", on the premise that a widget-style module has none. aether-ui's ui module has four (window, render_to, record, nav_page) beside ~50 _ctx-first containers (vstack, hstack, scrollview, splitview, …), so the rule's own comment ("a widget-style DSL module has no builders and is never flagged") does not hold for the largest DSL consumer in the org.

What

A fourth condition: the callee must yield no value. A container returns the handle its trailing block runs inside — that return value is the block's _ctx — while a block setter records config and returns nothing, which is precisely why its own trailing block can never be entered. That is the semantic line, not a heuristic:

• every aeb setter (rspec, minitest, env, gem_dep, rbenv, …) is void → still flagged;
• every aether-ui container returns a handle → not flagged;
• aether-ui's own void _ctx-first functions (bg_color, font_size, tooltip, …) are setters and are never written with a trailing block in that tree — and would now be flagged if they were, which is the rule working.

The void check runs on the already-resolved symbol (fn_yields_no_value, factored out of call_yields_no_value): the existing helper re-looks the callee up by bare name, which a mod.name call does not resolve, so it answered "yields a value" for every qualified call.

Verified

• tests/integration/setter_in_builder_position: the three existing cases plus LEGIT C (uimod: a builder beside handle-returning containers — uimod.window("app") { vstack(10) { label("hello") } } compiles) and MISUSE B (uimod.font_size(14) { … }, the module's void setter, still errors). 5/5.
• All 260 .ae sources under aether-ui examples/, apps/, tests/ type-check with this build (aetherc --check --lib .), against 1 before (every app).