emitter: a child that fails to construct reports its own error, not the reconciler's

[shreeve]

A child component that fails to construct is caught, reported, and left as null, but the setups for what it holds — a keyed for, an if, a text binding — still ran. They ran against an anchor that was never placed, and __reconcile's TypeError: undefined is not an object (evaluating 'anchor.parentNode') replaced the real error. The first error a newcomer meets, a misspelled name in a prop, came out as that.

Box contentOffsetY: nosuchname      # an undefined name
  for item, n in @items
    Text key: n, "#{item}"

On main this throws the TypeError; here it throws ReferenceError: nosuchname is not defined with the stack at the prop.

Cause and fix (src/emitter.js, renderChildComponent). The setups that belong to a child's projected content now run inside if (child) { … }. A child component made inside that content keeps its own setup step, which already checks itself, so it never sticks mid-mount. __reconcile refuses, by its own message, a list whose anchor was never placed; it does not fire here.

The DOM had the same masking. It appears only when the name is a prop of a child component that holds the list: on a plain div the throw aborts the whole mount before any list code runs.

Tests (all fail on main): in test/ui/components.test.js, two broken children kept to their placeholders, each reported as its own ReferenceError, with later state changes and unmount clean, plus an onError boundary around a child that holds a keyed for; in packages/tui/test.rip, the example above makes renderToString throw the ReferenceError and print nothing.

Corpus unchanged (0 written). dist/ regenerated. bun run test:all: 29 lanes, 12,668 tests.