Full rewrite of the demo's main thread from vanilla JS classes-with-DOM to
Svelte 5 components (runes mode), with a typed worker contract. Driven by an
internal code-quality audit that flagged a 380-line god function, dual sources
of truth for button-disabled state, drift between worker request/response
shapes, and a silent mobile belt-width bug.

Structural changes:

- Typed worker contract via discriminated unions (lib/types.ts). Single
  canonical Command shape — symbol === null means "keep" everywhere — replacing
  the dual KEEP-Symbol vs null impedance mismatch.
- MachineTab.svelte orchestrates per-engine state. executionMode is $state and
  every UI flag (panel/apply/take-control visibility, button-disabled) is a
  $derived line. The old applyMode switch and per-handler reset dance are gone.
- One <MachineTab engine={...}> instantiated per active tab; switching tabs
  unmounts/remounts (via {#key activeEngine}) which cleanly tears down the
  worker, timers, and CodeMirror instance.
- Editor lazy-loaded as a separate Vite chunk (~540 KB / 180 KB gz) so initial
  paint ships ~24 KB gz instead of ~200 KB gz; tape + control panel are visible
  immediately while CodeMirror loads in parallel.
- Lezer-based syntax preflight in the editor (markers before Load) plus
  boost-99 priority for machine-namespace identifiers in autocomplete.
- popstate listener so browser back/forward updates the active tab.
- Mobile belt width fixed: CSS owns geometry, JS only owns indexing counts.
  Three responsive tiers (≤480px / ≤768px / desktop) replace the single
  phone-tuned breakpoint.
- Surface tokens (--surface-*, --editor-bg) consolidate the duplicated
  panel-recipe CSS and align the log panel with CodeMirror's oneDark surface.
- Favicon repainted as a head cell (yellow doubled border + ▲ glyph below).

Tooling: TypeScript (strict), ESLint flat config (typescript-eslint +
eslint-plugin-svelte), svelte-check, Vite 5 + @sveltejs/vite-plugin-svelte 4,
svelte-codemirror-editor for the editor wrapper. Prettier and Vitest are
deferred — tests can land in a follow-up.

Multi-tape Turing display is tracked as
https://github.com/mellonis/machines-demo/issues/1; the worker now asserts
single-tape explicitly rather than silently dropping tapes ≥ 1.

Note: workerLive isn't reset on the worker-timeout path. Currently masked
because halted=true dominates the disabled-state gates; worth re-checking if
those gates ever loosen.