Architecture
- mirror-machine on the main thread is the source of truth for tape
  rendering during stepping; worker `stepped` responses no longer carry
  tapes (deterministic replay). Worker still sends full tape on `built`,
  `ran`, and `error` (partial state on mid-run failure).
- `Tape.svelte` reads `turing.Tape.viewport` directly via a new
  `setFromTape(tape, delta?, animate?)` API; no more snapshot shape
  conversion in the UI. Library guarantees `viewport.length === VIEWPORT_WIDTH`
  (mirror sets it via setter — workaround for upstream issue #95).
- Drop `blank` field from `TapeSnapshot`; UI sources it from the
  alphabet via `tape.alphabet.blankSymbol`.

Component split (MachineView 1037 -> 761 lines)
- `TapesStack.svelte` — multi-tape stack + head-thread connector,
  imperative API (setFromTape/clearAll/setTransitionsEnabled).
- `Toolbar.svelte` — Build/Step/Run/Stop + with-pause + examples
  dropdown; owns its own outside-click handling.
- Rename `MachineTab.svelte` -> `MachineView.svelte` (the file is
  the body shown when an engine tab is active; the tabs strip lives
  in App.svelte).
- Rename `runner.ts` -> `machineRunner.ts`, `worker.ts` -> `machineWorker.ts`
  (match exported class names).
- Extract numeric caps to new `lib/caps.ts`.

Worker contract
- Rename request/response: `load`/`loaded` -> `build`/`built` (matches
  the Build button rename).
- `runner.load()` -> `runner.build()`; internal handler names
  (`doLoad`, `loadDisabled`, `reloadWorker`) kept.
- Drop dead fields: `BuiltResponse.{stepsApplied, nextCommands}`,
  `RanResponse.halted`, `ErrorResponse.stack`.
- `WorkerError` class carries partial `tapes` from error responses so
  `failHalted` can mirror the state where execution stuck.

User-facing
- Build button replaces Load (hammer icon).
- "with pause" + interval input hidden in RUNNING_AUTO/RUNNING_CONTINUOUS.
- Step/Run stay enabled in HALTED (reload-from-code on entry restarts).
- Stop button next to Run during RUNNING_STEP/RUNNING_AUTO.
- DEMO mode runs `selectedExample.code` (canonical), not editor `code`.
- Code-changed warning fires once per running session (not per keystroke).
- Log: `<hr>` separator before each Build / first-Step / Run; per-tape
  groups with `Tape N:` header + indented `- blank/alphabet/tape` rows;
  paper-style shorthand `(<sym>/<L|R|S>)` after verbose description;
  command tense (kept/wrote/moved/stayed).

No UI substitution of alphabet symbols
- UI renders the literal user-chosen blank glyph; bundled examples
  switched to `'␣'` as the actual blank. CSS sugar
  (`.cell.blank`, `.cp-btn.blank`) gives blanks a dim visual hint
  without overloading any specific character — alphabets are free
  to include any glyph including `'␣'`.

CSS / tooling
- Native CSS nesting throughout (no preprocessor).
- ES2023 target (for `Array.prototype.with`).
- Favicon + header title use first-tape blue (`--accent`).
- Drop dead CSS token `--head-dim`; remove redundant `var()` fallbacks.

Naming consistency
- `cmds`/`cmd` -> `commands`/`command`; `mv` -> `movement`; `arr`,
  `cb`, `applyTimer` -> `array`, `callbacks`, `applyTimeoutId`;
  `entries` -> `logEntries`; `onclear`/`onreset`/`onclick` (custom
  prop) -> camelCase; `flashTimer` -> `flashTimeoutId`;
  `describeOne` -> `describeAppliedCommand`; `stepEntry`+`appliedEntry`
  -> single `commandsEntry(commands, application, colors?)`;
  `setFromSnapshot` -> `setFromTape`; `win` -> `viewport`.
- New `Alphabets` type alias (`readonly (readonly string[])[]`).
- Drop dead `IconButton.svelte` -> repurposed as corner-overlay icon
  button (used by Editor reset + Log clear).
- Drop dead `SYNTHETIC_ALPHABETS` constant; `alphabets` initialises
  to `[]` and resets to `[]` on error.

Documentation
- CLAUDE.md rewritten — architecture tree, MachineView overview,
  Tape section, multi-tape section, conventions, caps. Adds
  upstream-bug-workaround convention with running list (currently
  just turing-machine-js#95).
- README architecture tree synced.

Bumps version to 1.0.0-alpha.3.