docs: TodoMVC → production instruction set (standalone planning artifact)

[intendednull]

What this is

A single, self-contained report to hand a fresh session that will plan the
production work needed to bring a real, interactive, styled (TodoMVC-class) UI to
Buiy. It is the consolidated output of an exploratory prototype effort done on a
separate throwaway branch (not included in this PR, not for merge).

This PR adds one file and no code:
docs/reports/2026-06-18-todomvc-to-production-instruction-set.md.

What it contains (hold-nothing-back)

• The split — real buiy_core library work vs throwaway prototype scaffolding.
• The 3 real buiy_core bugs the prototype surfaced (none caught by existing
  tests): picking AABB-testing the parent-relative ResolvedLayout.position as
  if absolute (while the render uses the absolute GlobalTransform, "pillar 5") —
  which made every nested/centered widget unclickable; text_commit skipping a
  reshape on async font load — which crashed every DefaultPlugins app ~9 s
  in; and the still-unfixed TextSync clobber of a focused editor's typed
  content. Each with root cause, the prototype fix, and how to do it right in
  production.
• Features — the A11yToggled accessible-state seam + A11yRole::Checkbox, the
  Checkbox widget, keyboard activation, and the validated design call
  (A11yToggled as the single source of truth for checked-ness).
• Styling reality — exactly what the live draw path paints (flat: fills +
  uniform radius + text; no shadows/borders/gradients yet) and the theme-token model.
• Widget-catalog gaps — Button renders no visible label (and the picking
  hit-test interaction that forces co-located text), widget-state-in-bundle, the
  prelude gap.
• The verification blind spot — no tier runs a real DefaultPlugins app with
  real winit input, which is what hid all three bugs; the highest-leverage thing
  to add.
• An ordered production work breakdown (A→F) and the dev-environment pitfalls.

Everything needed to understand and act on it is inline; the only external
pointers are Buiy source-file paths (where to make the changes).

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